
<PAGE>
 
    As filed with the Securities and Exchange Commission on April 15, 1999
 
                                                     Registration No. 333-
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                      SECURITIES AND EXCHANGE COMMISSION
                             Washington, DC 20549
 
                                ---------------
 
                                   FORM S-3
                            REGISTRATION STATEMENT
                                     Under
                          The Securities Act of 1933
 
                                ---------------
 
                                  ONEOK, INC.
            (Exact Name of Registrant as Specified in Its Charter)
 
               Oklahoma                              73-1520922
    (State or Other Jurisdiction of               (I.R.S. Employer
    Incorporation or Organization)             Identification Number)
 
                             100 West Fifth Street
                             Tulsa, Oklahoma 74103
                                (918) 588-7000
   (Address, Including Zip Code, and Telephone Number, Including Area Code,
                 of Registrant's Principal Executive Offices)
 
                                James C. Kneale
             Vice President, Chief Financial Officer and Treasurer
                                  ONEOK, Inc.
                             100 West Fifth Street
                             Tulsa, Oklahoma 74103
                                (918) 588-7000
(Name, Address, Including Zip Code, and Telephone Number, Including Area Code,
                             of Agent For Service)
 
                                  Copies to:
            Donald A. Kihle                       Robert A. Yolles
            Gable & Gotwals                  Jones, Day, Reavis & Pogue
   100 West Fifth Street, Suite 1000            77 West Wacker Drive
         Tulsa, Oklahoma 74103                 Chicago, Illinois 60601
            (918) 585-8141                         (312) 782-3939
 
                                ---------------
 
   Approximate date of commencement of proposed sale to the public: From time
to time after this Registration Statement becomes effective as determined by
market conditions and other factors.
 
   If the only securities being registered on this form are being offered
pursuant to dividend or interest reinvestment plans, please check the
following box. [_]
 
   If any of the securities being registered on this form are to be offered on
a delayed or continuous basis pursuant to Rule 415 under the Securities Act of
1933, other than securities offered only in connection with dividend or
interest reinvestment plans, check the following box. [X]
 
   If this form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, please check the following
box and list the Securities Act registration statement number of the earlier
effective registration statement for the same offering. [_]
 
   If this form is a post-effective amendment filed pursuant to Rule 462(c)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering. [_]
 
   If delivery of the prospectus is expected to be made pursuant to Rule 434,
please check the following box. [_]
 
                        CALCULATION OF REGISTRATION FEE
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<TABLE>
<CAPTION>
                                              Proposed Maximum
            Title Of Each Class                  Aggregate         Amount Of
       Of Securities To Be Registered        Offering Price (1) Registration Fee
--------------------------------------------------------------------------------
<S>                                          <C>                <C>
Debt Securities (2)........................     $500,000,000        $139,000
</TABLE>
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(1) Estimated solely for the purpose of calculating the registration fee
    pursuant to Rule 457(o) under the Securities Act. In no event will the
    aggregate initial offering price of all Debt Securities issued from time
    to time pursuant to this Registration Statement exceed $500,000,000. If
    any Debt Securities are issued at an original issue discount, then the
    offering price will be deemed to be the principal amount that results in
    an aggregate initial offering price of up to $500,000,000, less the dollar
    amount of any Debt Securities previously issued hereunder.
(2) There is also being registered hereunder an indeterminate principal amount
    of Debt Securities as may be issuable upon conversion, redemption,
    exchange or exercise of the Debt Securities registered hereunder,
    including pursuant to any applicable antidilution provisions.
   The Registrant hereby amends this Registration Statement on such date or
dates as may be necessary to delay its effective date until the Registrant
shall file a further amendment which specifically states that this
Registration Statement shall thereafter become effective in accordance with
Section 8(a) of the Securities Act of 1933 or until this Registration
Statement shall become effective on such date as the Commission, acting
pursuant to said Section 8(a), may determine.
 
                                ---------------
 
 
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<PAGE>
 
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+The information in this prospectus is not complete and may be changed. We may +
+not sell these securities until the registration statement filed with the     +
+Securities and Exchange Commission is effective. This prospectus is not an    +
+offer to sell these securities and it is not soliciting an offer to buy these +
+securities in any state where the offer or sale is not permitted.             +
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
       Subject to completion, Preliminary Prospectus dated April 15, 1999
 
                                  $500,000,000
 
                                  ONEOK, Inc.
 
                                Debt Securities
 
                                  -----------
 
  We may offer from time to time unsecured Debt Securities, which may be senior
notes or debentures or other unsecured evidences of indebtedness in one or more
series. The aggregate initial offering price of the Debt Securities that are
offered will not exceed $500,000,000. We will offer the Debt Securities in an
amount and on terms to be determined by market conditions at the time of the
offering.
 
  We will provide specific terms of the particular Debt Securities being
offered in supplements to this prospectus. You should read this prospectus and
any supplement carefully before you invest. This prospectus may not be used to
sell Debt Securities unless accompanied by a prospectus supplement.
 
                                  -----------
 
  Neither the Securities and Exchange Commission nor any state securities
commission has approved or disapproved of these securities or passed upon the
adequacy or accuracy of this prospectus. Any representation to the contrary is
a criminal offense.
 
                                  -----------
 
                 The date of this Prospectus is         , 1999.
<PAGE>
 
                               TABLE OF CONTENTS
 
<TABLE>
<S>                                                                          <C>
About this Prospectus.......................................................   2
Where You Can Find More Information.........................................   2
Forward-Looking Information.................................................   3
About ONEOK.................................................................   4
Use of Proceeds.............................................................   5
Ratio of Earnings to Fixed Charges..........................................   5
Description of Debt Securities..............................................   5
Plan of Distribution........................................................  14
Legal Matters...............................................................  15
Experts.....................................................................  15
</TABLE>
 
                             ABOUT THIS PROSPECTUS
 
   This prospectus is part of a registration statement that ONEOK ("we" or
"ONEOK") filed with the Securities and Exchange Commission using a "shelf"
registration process. Using this process, we may offer the Debt Securities
described in this prospectus in one or more offerings with a total initial
offering price of up to $500,000,000. This prospectus provides you with a
general description of the Debt Securities we may offer. Each time we offer
Debt Securities, we will provide a prospectus supplement and any pricing
supplement that will contain information about the specific terms of that
particular offering. The prospectus supplement or pricing supplement may also
add, update or change information contained in this prospectus. To obtain
additional information that may be important to you, you should read the
exhibits filed by us with this registration statement or our other filings with
the Securities and Exchange Commission. You also should read this prospectus
and any prospectus supplement or pricing supplement together with the
additional information described under the heading "Where You Can Find More
Information."
 
                      WHERE YOU CAN FIND MORE INFORMATION
 
   We file annual, quarterly and special reports, proxy statements and other
information with the Securities and Exchange Commission. You can read and copy
any materials we file with the Securities and Exchange Commission at its Public
Reference Room at 450 Fifth Street, N.W., Washington, D.C. 20549 and at its
regional offices located at Seven World Trade Center, New York, New York 10048
and at 500 West Madison Street, Chicago, Illinois 60661. You can obtain
information about the operations of the Securities and Exchange Commission
Public Reference Room by calling the Securities and Exchange Commission at 1-
800-SEC-0330. The Securities and Exchange Commission also maintains a Web site
that contains information we file electronically with the Securities and
Exchange Commission, which you can access over the Internet at
http://www.sec.gov. Our Common Stock is listed on the New York Stock Exchange
(NYSE: OKE), and you can obtain information about us at the offices of the New
York Stock Exchange, 20 Broad Street, New York, New York 10005.
 
   This prospectus is part of a registration statement we have filed with the
Securities and Exchange Commission relating to the Debt Securities. As
permitted by Securities and Exchange Commission rules, this prospectus does not
contain all of the information we have included in the registration statement
and the accompanying exhibits we file with the Securities and Exchange
Commission. You may refer to the registration statement and the exhibits for
more information about us and the Debt Securities. The registration statement
and the exhibits are available at the Securities and Exchange Commission's
Public Reference Room or through its Web site.
 
   The Securities and Exchange Commission allows us to "incorporate by
reference" the information we file with it, which means that we can disclose
important information to you by referring you to those documents. The
information we incorporate by reference is an important part of this
prospectus, and later
 
                                       2
<PAGE>
 
information that we file with the Securities and Exchange Commission will
automatically update and supersede some of this information. We incorporate by
reference the documents listed below, and any future filings we make with the
Securities and Exchange Commission under Section 13(a), 13(c), 14 or 15(d) of
the Securities Exchange Act of 1934 until we sell all the Debt Securities. The
documents we incorporated by reference are:
 
  .  our annual report on Form 10-K for the year ended August 31, 1998;
 
  .  our quarterly reports on Form 10-Q for the quarterly periods ended
     November 30, 1998 and February 28, 1999; and
 
  .  our current reports on Form 8-K dated September 24, 1998 (as amended on
     October 2, 1998), October 21, 1998, December 16, 1998, December 16,
     1998, January 25, 1999 (as amended on January 26, 1999), January 25,
     1999, February 8, 1999, February 16, 1999, February 23, 1999, February
     24, 1999, February 24, 1999, February 26, 1999, March 5, 1999, March 19,
     1999 and April 15, 1999.
 
   You may request a copy of these filings (other than an exhibit to the
filings unless we have specifically incorporated that exhibit by reference into
the filing), at no cost, by writing or telephoning us at the following address:
 
                                  ONEOK, Inc.
                             100 West Fifth Street
                             Tulsa, Oklahoma 74103
                       Attention: Chief Financial Officer
                           Telephone: (918) 588-7000
 
   You should rely only on the information incorporated by reference or
provided in this prospectus or any prospectus supplement. We have not
authorized anyone else to provide you with different information. We may only
use this prospectus to sell Debt Securities if it is accompanied by a
prospectus supplement. We are only offering these Debt Securities in states
where the offer is permitted. You should not assume that the information in
this prospectus or any applicable prospectus supplement is accurate as of any
date other than the date on the front of those documents.
 
                          FORWARD-LOOKING INFORMATION
 
   Some of the statements contained and incorporated in this prospectus are
forward-looking statements within the meaning of the Private Securities
Litigation Reform Act of 1995. The forward-looking statements relate to
anticipated financial performance, management's plans and objectives for future
operations, business prospects, outcome of regulatory proceedings, market
conditions, the timing of the consummation of our proposed merger with
Southwest Gas Corporation and other matters. The Private Securities Litigation
Reform Act of 1995 provides a safe harbor for forward-looking statements in
various circumstances. The following discussion is intended to identify
important factors that could cause future outcomes to differ materially from
those set forth in the forward-looking statements.
 
   Forward-looking statements include the items identified in the preceding
paragraph, the information concerning possible or assumed future results of
operations and other statements contained or incorporated in this prospectus
identified by words such as "anticipate," "estimate," "expect," "intend,"
"believe," "projection" or "goal."
 
   You should not place undue reliance on the forward-looking statements. They
are based on known and unknown risks, uncertainties and other factors that may
cause our actual results, performance or achievements to be materially
different from any future results, performance or achievements expressed or
implied by the forward-looking statements. Those factors may affect our
operations, markets, products, services and prices. In addition to any
assumptions and other factors referred to specifically in connection with the
forward-looking statements, factors that could cause our actual results to
differ materially from those contemplated in any forward-looking statement
include, among others, the following:
 
                                       3
<PAGE>
 
  .  industry capacity;
 
  .  changes in technology;
 
  .  regulatory delays or conditions imposed by regulatory bodies in
     approving the proposed merger;
 
  .  the effects of weather and other natural phenomena on sales and prices;
 
  .  increased competition from other energy suppliers as well as alternative
     forms of energy;
 
  .  the capital intensive nature of our business;
 
  .  further deregulation, or "unbundling," of the natural gas business;
 
  .  economic climate and growth in the geographic areas in which we do
     business;
 
  .  the uncertainty of gas and oil reserve estimates;
 
  .  the timing and extent of changes in commodity prices for natural gas,
     electricity, crude oil and natural gas liquids;
 
  .  conditions of capital markets and equity markets, including conditions
     that may affect our ability to finance the proposed merger with
     Southwest Gas;
 
  .  Year 2000 issues;
 
  .  the effects of changes in governmental policies and regulatory actions,
     including income taxes, environmental compliance and authorized rates;
     and
 
  .  the other factors listed in the reports we have filed and may file with
     the Securities and Exchange Commission, which are incorporated by
     reference.
 
   Other factors and assumptions not identified above were also involved in the
making of the forward-looking statements. The failure of those assumptions to
be realized, as well as other factors, may also cause actual results to differ
materially from those projected. We have no obligation and make no undertaking
to update publicly or revise any forward-looking statements.
 
                                  ABOUT ONEOK
 
   ONEOK and its subsidiaries engage in several aspects of the energy business.
We purchase, gather, compress, transport and store natural gas for distribution
to consumers. We transport gas for others and lease pipeline capacity to others
for their use in transporting gas. We drill for and produce gas and oil,
extract and sell natural gas liquids, and engage in the gas marketing business.
In addition, we lease and operate a headquarters office building (leasing space
we do not use to others) and own and operate a related parking facility. As a
regulated natural gas utility, we distribute natural gas to approximately 1.4
million customers in the states of Oklahoma and Kansas.
 
   On December 14, 1998, we announced that we had entered into an Agreement and
Plan of Merger providing for the acquisition by merger of Southwest Gas for
cash of approximately $864 million. We will be the survivor of the proposed
merger transaction. The proposed merger is subject to customary conditions,
including approval by the shareholders of Southwest Gas and the receipt of
governmental and other authorizations, and is expected to close later in 1999.
We believe that the requisite regulatory approvals will be received, but we are
uncertain when we will receive those approvals. It is possible that regulatory
approvals that are obtained will contain conditions or limitations that will
adversely affect the results of operations of the combined company. Unaudited
pro forma combined condensed financial data giving effect to the proposed
merger are included in our current report on Form 8-K dated April 15, 1999,
which is incorporated herein by reference. We expect to issue about $660
million of debt and $250 million of trust preferred securities in order to
finance the proposed merger.
 
                                       4
<PAGE>
 
                                USE OF PROCEEDS
 
   Unless we inform you otherwise in the prospectus supplement, we will use the
net proceeds from the sale of the Debt Securities offered by this prospectus
for general corporate purposes. These purposes may include repayment and
refinancing of debt, acquisitions (including our acquisition of Southwest Gas),
working capital, capital expenditures and repurchases and redemptions of
securities. Pending any specific application, we may initially invest funds in
short-term marketable securities or apply them to the reduction of short-term
indebtedness.
 
                       RATIO OF EARNINGS TO FIXED CHARGES
 
   Our ratio of earnings to fixed charges for each of the periods shown is as
follows:
 
<TABLE>
<CAPTION>
                                    For the Years Ended August 31,
      For the Six Months Ended     ----------------------------------------------
         February 28, 1999         1998      1997      1996      1995      1994
      ------------------------     ----      ----      ----      ----      ----
      <S>                          <C>       <C>       <C>       <C>       <C>
               6.58x               5.50x     3.51x     3.28x     2.70x     2.52x
</TABLE>
 
   We have computed the ratios of earnings to fixed charges by dividing
earnings by fixed charges. For this purpose, "earnings" consist of net income
plus fixed charges and income taxes. "Fixed charges" consist of interest
charges, that portion of annual operating lease expense we have deemed to
represent the interest factor and the amortization of debt issue costs.
 
                         DESCRIPTION OF DEBT SECURITIES
 
   The following description states the general terms and provisions of our
unsecured Debt Securities. In this prospectus, "Debt Securities" means the
debentures, notes, bonds and other evidences of indebtedness that we will issue
under an Indenture that we entered into with Chase Bank of Texas, National
Association, as Trustee, on September 24, 1998. Each prospectus supplement that
we provide when we offer Debt Securities will describe the specific terms of
the Debt Securities offered through that prospectus supplement and any general
terms outlined in this section that will not apply to those Debt Securities.
 
   We have summarized various terms and provisions of the Indenture in this
section. The summary is not complete. We also have filed the form of the
Indenture as an exhibit to this registration statement. You should read the
form of Indenture for additional information before you buy any Debt
Securities. The Indenture will be qualified under the Trust Indenture Act of
1939. You should refer to the Trust Indenture Act for provisions that apply to
the Debt Securities. This summary also is subject to and qualified by reference
to the description of the particular terms of the Debt Securities described in
the applicable prospectus supplement or supplements and pricing supplement or
supplements. Capitalized terms used but not defined in this summary have the
meanings specified in the Indenture.
 
   The Debt Securities will be our unsecured obligations and will rank equally
with any of our other senior, unsecured debt. Debt Securities issued under the
Indenture will be issued as part of a series that has been established pursuant
to a supplemental indenture or other corporate action designating the specific
terms of the series of Debt Securities. A prospectus supplement will describe
these terms and will include, among other things, the following:
 
  .  the title of the Debt Securities;
 
  .  the total principal amount of the Debt Securities and the percentage of
     their principal amount at which we will issue the Debt Securities;
 
  .  the date or dates on which the principal of the Debt Securities will be
     payable;
 
  .  the interest rate, the method for determining the interest rate (if the
     interest rate is variable), the date from which interest will accrue,
     interest payment dates and record dates for interest payments;
 
                                       5
<PAGE>
 
  .  the place or places where payments on the Debt Securities will be made,
     where holders may surrender their Debt Securities for transfer or
     exchange and where to serve notices or demands;
 
  .  any provisions for optional redemption or early repayment;
 
  .  any provisions that would obligate us to redeem, purchase or repay Debt
     Securities;
 
  .  whether payments on the Debt Securities will be payable by reference to
     any index, formula or other method;
 
  .  any deletions from, changes of or additions to the events of default or
     covenants described in this prospectus;
 
  .  the portion of the principal amount of Debt Securities that will be
     payable if the maturity is accelerated, if other than the entire
     principal amount;
 
  .  any additional means of defeasance of the Debt Securities, any
     additional conditions or limitations to defeasance of the Debt
     Securities or any changes to those conditions or limitations;
 
  .  any provisions granting special rights to holders of the Debt Securities
     upon the occurrence of events identified in the prospectus supplement;
 
  .  if other than the Trustee, the designation of any Paying Agent or
     Security Registrar for the Debt Securities; and the designation of any
     transfer or other agents or depositories for the Debt Securities;
 
  .  whether we will issue the Debt Securities in individual certificates to
     each holder or in the form of temporary or permanent global securities
     that a depository will hold on behalf of holders;
 
  .  the denominations in which we will issue the Debt Securities or in which
     they may be owned, if other than $1,000 or any integral multiple of
     $1,000;
 
  .  whether and in what circumstances any additional amounts may be payable
     on the Debt Securities to foreign holders; and
 
  .  any other terms or conditions that are consistent with the Indenture.
 
   We may sell the Debt Securities at a discount (which may be substantial)
below their stated principal amount. These discounted Debt Securities may bear
no interest or interest at a rate that at the time of issuance is below market
rates. We will describe in the prospectus supplement any material United States
federal income tax consequences and other special considerations.
 
Restrictive Covenants
 
   We have agreed to two principal restrictions on our activities for the
benefit of holders of the Debt Securities. The restrictive covenants summarized
below will apply to a series of Debt Securities (unless waived or amended) as
long as any of those Debt Securities are outstanding, unless the prospectus
supplement for the series states otherwise. We have used in this summary
description capitalized terms that we have defined below under "--Glossary." In
this description of the covenants only, all references to "us" or "we" mean
ONEOK and our principal subsidiaries, unless the context clearly indicates
otherwise. Our principal subsidiaries are those that own or lease a Principal
Property.
 
   Other than the restrictions on Liens and Sale/Leaseback transactions
described below, the Indenture and the Debt Securities do not contain any
covenants or other provisions designed to protect holders of any Debt
Securities in the event we participate in a highly leveraged transaction. The
Indenture and the Debt Securities also do not contain provisions that give
holders of the Debt Securities the right to require us to repurchase their
securities in the event of a decline in our credit ratings resulting from a
takeover, recapitalization or similar restructuring or otherwise.
 
                                       6
<PAGE>
 
   For these purposes, "debt" includes all notes, bonds, debentures or similar
evidences of obligations for borrowed money.
 
 Limitation on Liens
 
   We have agreed that we will issue, assume or guarantee debt for borrowed
money secured by any Lien upon a Principal Property or shares of stock or debt
of any principal subsidiary only if we secure the Debt Securities equally and
ratably with or prior to the debt secured by that Lien. If we secure the Debt
Securities in this manner, we have the option to secure any of our other debt
or obligations equally and ratably with or prior to the debt secured by the
Lien and, accordingly, equally and ratably with the Debt Securities. This
covenant has exceptions that permit:
 
  .  Liens that existed on the date we first issued a series of Debt
     Securities;
 
  .  Liens on any Principal Property or Restricted Securities of any entity
     existing at the time we merge or consolidate with that entity or acquire
     its property or at the time the entity becomes a principal subsidiary;
 
  .  Liens on any Principal Property existing at the time we acquire that
     Principal Property so long as the Lien does not extend to any of our
     other Principal Property;
 
  .  Liens on any Principal Property, and any Lien on the shares of stock of
     any principal subsidiary formed for the purpose of acquiring that
     Principal Property, either:
 
    .  securing all or part of the cost of acquiring, constructing,
       improving, developing or expanding the Principal Property that was
       incurred before, at or within 12 months after the latest of the
       acquisition or completion of the assets or their commencing
       commercial operation; or
 
    .  securing debt to finance the purchase price of the Principal
       Property or the cost of constructing, improving, developing or
       expanding the assets that was incurred before, at or within 12
       months after the latest of the acquisition or completion of the
       assets or their commencing commercial operation;
 
  .  Liens on any Principal Property or Restricted Securities to secure debt
     owed to us;
 
  .  Liens securing industrial development, pollution control or other
     revenue bonds of a government entity;
 
  .  Liens arising in connection with a project financed with, and securing,
     Non-Recourse Indebtedness;
 
  .  Statutory or other Liens arising in the ordinary course of business and
     relating to amounts that are not delinquent or remain payable without
     penalty or that we are contesting in good faith;
 
  .  Liens (other than Liens imposed by ERISA) on our property incurred or
     required in connection with workmen's compensation, unemployment
     insurance and other social security legislation;
 
  .  Liens securing taxes that remain payable without penalty or that we are
     contesting in good faith if we believe we have adequate reserves for the
     taxes in question;
 
  .  Rights that any governmental entity may have to purchase or order the
     sale of any of our property upon payment of reasonable compensation;
 
  .  Rights that any governmental entity may have to terminate any of our
     franchises, licenses or other rights or to regulate our property and
     business;
 
  .  Liens that we do not assume or on which we do not customarily pay
     interest and that exist on real estate or other rights we acquire for
     sub-station, measuring station, regulating station, gas purification
     station, compressor station, transmission line, distribution line or
     right-of-way purposes;
 
                                       7
<PAGE>
 
  .  Easements or reservations in our property for roads, pipelines, gas
     transmission and distribution lines, electric light and power
     transmission and distribution lines, water mains and other similar
     purposes and zoning ordinances, regulations and restrictions that do not
     impair the use of the property in the operation of our business;
 
  .  Any extensions, renewals, substitutions or replacements of the above-
     described Liens or any debt secured by these Liens if both:
 
    .  the amount of debt secured by the new Lien and not otherwise
       permitted does not exceed the principal amount of debt so secured at
       the time of the renewal or refunding; and
 
    .  the new Lien is limited to the property (plus any improvements)
       secured by the original Lien.
 
   In addition, without securing the Debt Securities as described above, we may
issue, assume or guarantee debt that this covenant would otherwise restrict in
a total principal amount that, when added to all of our other outstanding debt
that this covenant would otherwise restrict and the total amount of
Attributable Debt outstanding for Sale/Leaseback Transactions, does not exceed
a "basket" equal to 15% of Consolidated Net Tangible Assets. When calculating
this total principal amount, we exclude from the calculation Attributable Debt
from Sale/Leaseback Transactions in connection with which we have purchased
property or retired debt as described below under "--Limitation on
Sale/Leaseback Transactions."
 
 Limitation on Sale/Leaseback Transactions
 
   We have agreed that we will enter into a Sale/Leaseback Transaction only if
at least one of the following applies:
 
  .  we could incur debt secured by a Lien on the Principal Property that is
     the subject of that Sale/Leaseback Transaction;
 
  .  the Attributable Debt subject to Sale/Leaseback Transaction would be in
     an amount permitted under the "basket" described above under "--
     Limitation on Liens";
 
  .  the proceeds of that Sale/Leaseback Transaction are used for our
     business and operations; or
 
  .  within the period ending 12 months after the closing of the
     Sale/Leaseback Transaction, we apply the net proceeds of the
     Sale/Leaseback Transaction to the voluntary retirement of any Debt
     Securities or Funded Indebtedness (other than Funded Indebtedness that
     we hold or that is subordinate in right of payment to any Debt
     Securities).
 
 Glossary
 
   "Attributable Debt" means, as to any particular lease under which any Person
is at the time liable, at any date as of which the amount thereof is to be
determined, the total net amount of rent required to be paid by that Person
under the lease during the remaining term thereof (excluding amounts required
to be paid on account of maintenance and repairs, services, insurance, taxes,
assessments, water rates and similar charges and contingent rents), discounted
from the respective due dates thereof at the weighted average of the rates of
interest (or yield to maturity, in the case of Debt Securities originally sold
at a discount) borne by the Debt Securities then outstanding under the
Indenture, compounded annually.
 
   "Consolidated Net Tangible Assets" means (1) the total amount of assets
(less applicable reserves and other properly deductible items) which under GAAP
would be included on a consolidated balance sheet of ONEOK and its subsidiaries
after deducting therefrom (A) all current liabilities, provided, however, that
there will not be deducted billings recorded as revenues deferred pending the
outcome of rate proceedings (less applicable income taxes thereon), if and to
the extent the obligation to refund the same has not been finally determined,
(B) appropriate allowance for minority interests in common stocks of
subsidiaries and (C) all goodwill, trade names, trademarks, patents,
unamortized debt discount and expense and other like intangibles,
 
                                       8
<PAGE>
 
which in each case under GAAP would be included on the consolidated balance
sheet, less (2) the amount which would be so included on the consolidated
balance sheet for investments (less applicable reserves) made in subsidiaries.
 
   "Funded Indebtedness" as applied to any Person, means all debt of that
Person maturing after, or renewable or extendible at that Person's option
beyond, 12 months from the date of determination.
 
   "Lien" means any lien, mortgage, pledge, encumbrance, charge or security
interest securing debt; provided, however, that the following types of
transactions will not be considered for purposes of this definition to result
in a Lien: (1) any acquisition by us of any property or assets subject to any
reservation or exception under the terms of which any vendor, lessor or
assignor creates, reserves or excepts or has created, reserved or excepted an
interest in oil, gas or any other mineral in place or the proceeds thereof, (2)
any conveyance or assignment whereby we convey or assign to any Person or
Persons an interest in oil, gas or any other mineral in place or the proceeds
thereof, (3) any Lien upon any property or assets either owned or leased by us
or in which we own an interest that secures for the benefit of the Person or
Persons paying the expenses of developing or conducting operations for the
recovery, storage, transportation or sale of the mineral resources of the
property or assets (or property or assets with which it is unitized) the
payment to that Person or Persons of our proportionate part of the development
or operating expenses or (4) any hedging arrangements entered into in the
ordinary course of business, including any obligation to deliver any mineral,
commodity or asset in connection with the arrangement.
 
   "Non-Recourse Indebtedness" means, at any time, debt incurred after the date
of the Indenture by us in connection with the acquisition of property or assets
by us or the financing of the construction of or improvements on property,
whenever acquired, provided that, under the terms of that debt and pursuant to
applicable law, the recourse at that time and thereafter of the lenders with
respect to the debt is limited to the property or assets so acquired, or the
construction or improvements, including debt as to which a performance or
completion guarantee or similar undertaking was initially applicable to the
debt or the related property or assets if the guarantee or similar undertaking
has been satisfied and is no longer in effect.
 
   "Person" means any individual, corporation, partnership, limited liability
company, joint venture, association, joint-stock company, trust, unincorporated
organization or government or any agency or political subdivision thereof.
 
   "Principal Property" means any property located in the United States, except
any property that in the opinion of the Board of Directors of ONEOK is not of
material importance to the total business conducted by ONEOK and its
consolidated subsidiaries.
 
   "Sale/Leaseback Transaction" means any arrangement with any Person pursuant
to which we lease any Principal Property that has been or is to be sold or
transferred by us to that Person, other than (1) a lease for a term, including
renewals at the option of the lessee, of not more than three years or
classified as an operating lease under generally accepted accounting
principles, (2) leases between ONEOK and one of our principal subsidiaries or
between principal subsidiaries, (3) leases of a Principal Property executed by
the time of, or within 12 months after the latest of, the acquisition, the
completion of construction or improvement, or the commencement of commercial
operation, of the Principal Property and (4) the ground lease for ONEOK Plaza,
100 West Fifth Street, Tulsa, Oklahoma 74103.
 
Consolidation, Merger and Sale of Assets
 
   The Indenture generally permits a consolidation or merger between us and
another entity. It also permits the sale by us of all or substantially all of
our assets. We have agreed, however, that we will consolidate with or merge
into any entity or transfer or dispose of all or substantially all of our
assets to any entity only if:
 
  .  immediately after giving effect to the transaction, no default or event
     of default would have occurred and be continuing or would result from
     the transaction;
 
                                       9
<PAGE>
 
  .  we are the continuing corporation or, if we are not the continuing
     corporation, the resulting entity is organized and existing under the
     laws of any United States jurisdiction and assumes the due and punctual
     payments on the Debt Securities and the performance of our covenants and
     obligations under the Indenture and the Debt Securities; and
 
  .  we provide the Trustee with a certificate and a legal opinion, each
     stating that the Indenture permits the transaction.
 
   If we engage in any of these transactions that result in any Principal
Property or Restricted Securities becoming subject to any Lien and unless we
are otherwise able to create that Lien, the Indenture provides that the Debt
Securities (so long as those Debt Securities are entitled to the protection of
the "Limitation on Liens" covenant) will be secured to at least the same extent
as the debt that would become secured by the Lien as a result of the
transaction.
 
Events of Default
 
   Unless we inform you otherwise in the prospectus supplement, the following
are events of default for a series of Debt Securities:
 
  .  our failure to pay interest on that series of Debt Securities for 30
     days after it becomes due and payable;
 
  .  our failure to pay principal of or any premium on that series of Debt
     Securities when due;
 
  .  our failure to comply with any of our covenants or agreements for that
     series of Debt Securities or in the Indenture (other than an agreement
     or covenant that we have included in the Indenture solely for the
     benefit of less than all series of Debt Securities) for 60 days after
     the Trustee or the holders of at least 25% in principal amount of all
     outstanding Debt Securities affected by that failure provide written
     notice to us;
 
  .  the default under any agreement under which we have at the time
     outstanding debt in excess of $15,000,000 and, if that debt has not
     already matured, it has been accelerated and the acceleration is not
     rescinded within 30 days after we receive notice from the Trustee or the
     holders of at least 25% in principal amount of all outstanding Debt
     Securities of a series so long as, prior to the entry of judgment in
     favor of the Trustee for payment of the Debt Securities of that series,
     we do not cure the default, or the default under the agreement has not
     been waived;
 
  .  various events involving the bankruptcy, insolvency or reorganization of
     ONEOK; or
 
  .  any other event of default provided for that series of Debt Securities.
 
   A default under one series of Debt Securities will not necessarily be a
default under another series. The Trustee may withhold notice to the holders of
a series of Debt Securities of any default or event of default (except in any
payment on that series of Debt Securities) if the Trustee considers it in the
interest of the holders of that series of Debt Securities to do so.
 
   If an event of default for any series of Debt Securities occurs and is
continuing, the Trustee or the holders of at least 25% in principal amount of
the outstanding Debt Securities of the series affected by the default (or, in
some cases, 25% in principal amount of all Debt Securities affected, voting as
one class) may require us to pay on an accelerated basis the principal of and
all accrued and unpaid interest on those Debt Securities. The holders of a
majority in principal amount of the outstanding Debt Securities of the series
affected by the default (or of all Debt Securities affected, voting as one
class) may in some cases rescind this accelerated payment requirement.
 
   If an event of default occurs and is continuing, the Trustee must use the
degree of care and skill of a prudent man in the conduct of his own affairs.
The Trustee will become obligated to exercise any of its powers
 
                                       10
<PAGE>
 
under the Indenture at the request of any of the holders of any Debt Securities
only after those holders have offered the Trustee indemnity reasonably
satisfactory to it.
 
   The holders of a majority in principal amount of Debt Securities of any
series have the rights to waive past defaults under the Indenture that relate
to that series except for a default in the payment on the Debt Securities or a
provision that can only, under the Indenture, be modified or amended if all
holders that are affected consent.
 
   In most cases, holders of a majority in principal amount of the outstanding
Debt Securities of a series (or of all Debt Securities affected, voting as one
class) may direct the time, method and place of:
 
  .  conducting any proceeding for any remedy available to the Trustee; and
 
  .  exercising any trust or power conferred on the Trustee.
 
   The Indenture requires us to file each year with the Trustee a written
statement as to our compliance with the covenants contained in the Indenture.
 
Modification and Waiver
 
   We may amend or supplement the Indenture if the holders of a majority in
principal amount of the outstanding Debt Securities of all series that the
amendment or supplement affects (acting as one class) consent to it. Without
the consent of the holder of each Debt Security affected, however, no
modification may:
 
  .  reduce the principal of the Debt Security or change its stated maturity;
 
  .  reduce the rate of or change the time for payment of interest on the
     Debt Security;
 
  .  reduce any premium payable on the redemption of the Debt Security or
     change the time at which the Debt Security may or must be redeemed;
 
  .  change any obligation to pay additional amounts on the Debt Security;
 
  .  impair the holder's right to institute suit for the enforcement of any
     payment on the Debt Security;
 
  .  impair the holder's right to convert or exchange any Debt Security;
 
  .  reduce the percentage of principal amount of Debt Securities whose
     holders must consent to an amendment to or supplement of the Indenture;
 
  .  reduce the percentage of principal amount of Debt Securities necessary
     to waive compliance with some of the provisions of the Indenture; or
 
  .  modify provisions relating to amendment or waiver, except to increase
     percentages or to provide that other provisions of the Indenture cannot
     be amended or waived without the consent of each holder affected.
 
   We may amend or supplement the Indenture or waive any provision of it
without the consent of any holders of Debt Securities in various circumstances,
including:
 
  .  to provide for the assumption of our obligations under the Indenture and
     the Debt Securities by a successor;
 
  .  to add covenants that would benefit the holders of any Debt Securities
     or to surrender any of our rights or powers;
 
  .  to add events of default with respect to any Debt Securities;
 
  .  to change or eliminate any provisions only when there are no outstanding
     Debt Securities entitled to the benefit of those provisions;
 
  .  to provide any security for or guarantees of any series of Debt
     Securities;
 
                                       11
<PAGE>
 
  .  to provide for the form or terms of any series of Debt Securities;
 
  .  to appoint a successor Trustee or to provide for the administration of
     the trusts under the Indenture by more than one Trustee;
 
  .  to close the Indenture with respect to the authentication and delivery
     of additional series of Debt Securities;
 
  .  to cure any ambiguity, omission, defect or inconsistency that does not
     adversely affect the interests of the holders of outstanding Debt
     Securities of any series;
 
  .  to make any change to the extent necessary to permit or facilitate
     defeasance or discharge of any series of Debt Securities that does not
     adversely affect the interests of the holders of outstanding Debt
     Securities of any series; or
 
  .  to make any change that does not affect the rights of any holder.
 
   The holders of a majority in principal amount of the outstanding Debt
Securities may waive our obligations to comply with various covenants,
including those relating to (1) our obligation to secure the Debt Securities in
the event of mergers, consolidations and sales of assets, (2) corporate
existence and (3) the restrictions on Liens and Sale/Leaseback Transactions.
 
Defeasance
 
   When we use the term defeasance, we mean discharge from some or all of our
obligations under the Indenture. If we deposit with the Trustee funds or
government securities the maturing principal and interest of which is
sufficient to make payments on the Debt Securities of a series on the dates
those payments are due and payable, then, at our option, either of the
following will occur:
 
  .  we will be discharged from our obligations with respect to the Debt
     Securities of that series ("legal defeasance"); or
 
  .  we will no longer have any obligation to comply with the restrictive
     covenants under the Indenture and any other restrictive covenants that
     apply to that series of the Debt Securities, and the related events of
     default will no longer apply to us ("covenant defeasance").
 
   If we defease a series of Debt Securities, the holders of the Debt
Securities of the series affected will not be entitled to the benefits of the
Indenture, except for our obligations to pay additional amounts, if any, to
provide temporary Debt Securities, to register the transfer or exchange of Debt
Securities, to replace stolen, lost or mutilated Debt Securities or to maintain
paying agencies and hold moneys for payment in trust. In the case of covenant
defeasance, our obligation to pay principal, premium and interest on the Debt
Securities will also survive.
 
   Unless we inform you otherwise in the prospectus supplement, we will be
required to deliver to the Trustee an opinion of counsel that the deposit and
related defeasance would not cause the holders of the Debt Securities to
recognize income, gain or loss for federal income tax purposes. If we elect
legal defeasance, that opinion of counsel must be based upon a ruling from the
Internal Revenue Service or a change in law to that effect.
 
Governing Law
 
   New York law will govern the Indenture and the Debt Securities.
 
Trustee
 
   The Indenture contains limitations on the right of the Trustee, if it
becomes one of our creditors, to obtain payment of claims or to realize on
property received for those claims, as security or otherwise. The Trustee is
 
                                       12
<PAGE>
 
permitted to engage in other transactions with us. If, however, it acquires any
conflicting interest, it must eliminate that conflict or resign.
 
Form, Exchange, Registration and Transfer
 
   We will issue the Debt Securities in registered form, without interest
coupons. We will not charge a service charge for any registration of transfer
or exchange of the Debt Securities. We may, however, require the payment of any
tax or other governmental charge payable for that registration.
 
   Holders may exchange Debt Securities of any series for other Debt Securities
of the same series, the same total principal amount and the same terms but in
different authorized denominations in accordance with the Indenture. Holders
may present Debt Securities for registration of transfer at the office of the
security registrar or any transfer agent we designate. The security registrar
or transfer agent will effect the transfer or exchange when it is satisfied
with the documents of title and identity of the person making the request.
 
   We have appointed the Trustee as security registrar for the Debt Securities.
If a prospectus supplement refers to any transfer agents initially designated
by us, we may at any time rescind that designation or approve a change in the
location through which any transfer agent acts. We are required to maintain an
office or agency for transfers and exchanges in each place of payment. We may
at any time designate additional transfer agents for any series of Debt
Securities.
 
   In the case of any redemption, neither the security registrar nor the
transfer agent will be required to register the transfer or exchange of any
Debt Security either:
 
  .  during a period beginning 15 business days prior to the mailing of the
     relevant notice of redemption and ending on the close of business on the
     day of mailing of that notice; or
 
  .  if we have called the Debt Security for redemption in whole or in part,
     except the unredeemed portion of any Debt Security being redeemed in
     part.
 
Payment and Paying Agents
 
   Unless we inform you otherwise in a prospectus supplement, payments on the
Debt Securities will be made in U.S. dollars at the office of the Trustee. At
our option, however, we may make payments by check mailed to the holder's
registered address or, for global Debt Securities, by wire transfer. Unless we
inform you otherwise in a prospectus supplement, we will make interest payments
to the person in whose name the Debt Security is registered at the close of
business on the record date for the interest payment.
 
   Unless we inform you otherwise in a prospectus supplement, we will designate
the Trustee as our paying agent for payments on Debt Securities issued under
the Indenture. We may at any time designate additional paying agents or rescind
the designation of any paying agent or approve a change in the office through
which any paying agent acts.
 
   Subject to the requirements of any applicable abandoned property laws, the
Trustee and paying agent will pay to us upon written request any money they are
holding for payments on the Debt Securities that remain unclaimed for two years
after the date upon which that payment has become due. After payment to us,
holders entitled to the money must look to us for payment. In that case, all
liability of the Trustee or paying agent with respect to that money will cease.
 
Book-Entry Debt Securities
 
   We may issue the Debt Securities of a series in the form of one or more
global Debt Securities that would be deposited with a depositary or its nominee
identified in the prospectus supplement. We may issue global Debt Securities in
either temporary or permanent form. We will describe in the prospectus
supplement the
 
                                       13
<PAGE>
 
terms of any depositary arrangement and the rights and limitations of owners of
beneficial interests in any global Debt Security.
 
                              PLAN OF DISTRIBUTION
 
   We may sell the Debt Securities offered under this prospectus through
underwriters, agents or dealers or directly to purchasers.
 
Underwriters
 
   The applicable prospectus supplement will identify any agents or
underwriters and describe their compensation (including underwriting discount).
The prospectus supplement will also describe other terms of the offering,
including any discounts or concessions allowed or reallowed or paid to dealers
and any securities exchanges on which the offered Debt Securities may be
listed.
 
   The distribution of Debt Securities under this prospectus may occur from
time to time in one or more transactions at a fixed price or prices, which may
be changed, at market prices prevailing at the time of sale, at prices related
to those prevailing market prices or at negotiated prices.
 
Agents and Direct Sales
 
   If the applicable prospectus supplement indicates, we will authorize dealers
or our agents to solicit offers by various institutions to purchase offered
Debt Securities from us pursuant to contracts that provide for payment and
delivery on a future date. We must approve all institutions, but they may
include, among others:
 
  .  commercial and savings banks;
 
  .  insurance companies;
 
  .  pension funds;
 
  .  investment companies; and
 
  .  educational and charitable institutions.
 
   The institutional purchaser's obligations under a contract will be subject
only to the condition that the purchase of the offered Debt Securities at the
time delivery is allowed by any laws that govern the purchaser. The dealers and
our agents will not be responsible for the validity or performance of the
contracts.
 
General Information
 
   Underwriters, dealers and agents participating in a sale of Debt Securities
may be deemed to be underwriters as defined in the Securities Act of 1933, and
any discounts and commissions received by them and any profit realized by them
on resale of the Debt Securities may be deemed to be underwriting discounts and
commissions under the Securities Act. We may have agreements with the agents,
underwriters and dealers to indemnify them against various civil liabilities,
including liabilities under the Securities Act, or to contribute to payments
that the agents, underwriters or dealers may be required to make as a result of
those civil liabilities.
 
   Unless we indicate differently in a prospectus supplement, we will not list
the Debt Securities on any securities exchange. If we sell a security offered
under this prospectus to an underwriter for public offering and sale, the
underwriter may make a market for that security but is not obligated to do so.
Therefore, we cannot give any assurances to you concerning the liquidity of any
security offered under this prospectus.
 
   Agents and underwriters and their affiliates may be customers of, engage in
transactions with, or perform services for us or our subsidiary companies in
the ordinary course of business.
 
                                       14
<PAGE>
 
                                 LEGAL MATTERS
 
   The validity of the Debt Securities will be passed upon for ONEOK by Gable &
Gotwals, Tulsa, Oklahoma. Any underwriters or agents will be advised about
other issues relating to any offering by Jones, Day, Reavis & Pogue, Chicago,
Illinois. Jones, Day, Reavis & Pogue will rely on Gable & Gotwals as to matters
of Oklahoma law. Jones, Day, Reavis & Pogue from time to time acts as counsel
to ONEOK.
 
                                    EXPERTS
 
   The consolidated financial statements of ONEOK and its subsidiaries as of
August 31, 1998 and 1997, and for each of the years in the three-year period
ended August 31, 1998 have been incorporated by reference herein in reliance
upon the report of KPMG LLP, independent certified public accountants,
incorporated by reference herein, and upon the authority of said firm as
experts in accounting and auditing.
 
   The consolidated financial statements of Southwest Gas and its subsidiaries
as of December 31, 1998 and 1997, and for each of the years in the three-year
period ended December 31, 1998, incorporated by reference in ONEOK's current
report on Form 8-K dated April 15, 1999, incorporated by reference herein, have
been audited by Arthur Andersen LLP, independent public accountants, as
indicated in their report with respect thereto, and which are incorporated by
reference herein in reliance upon the authority of said firm as experts in
giving said report.
 
                                       15
<PAGE>
 
                                    PART II.
 
                     INFORMATION NOT REQUIRED IN PROSPECTUS
 
Item 14. Other Expenses of Issuance and Distribution.
 
   The following table sets forth the expenses in connection with the issuance
and distribution of the securities being registered, other than underwriting
discounts and commissions. All of the amounts shown are estimated, except the
Securities and Exchange Commission registration fee.
 
<TABLE>
      <S>                                                              <C>
      Securities and Exchange Commission registration fee............. $139,000
      Legal fees and expenses.........................................   35,000
      Printing and engraving..........................................   50,000
      Fees of accountants.............................................   40,000
      Fees of Trustee.................................................   15,000
      Rating Agencies Fees............................................  150,000
      Blue sky fees and expenses......................................    1,500
      Miscellaneous...................................................    9,500
                                                                       --------
                                                                       $440,000
                                                                       ========
</TABLE>
 
Item 15. Indemnification of Directors and Officers.
 
   The Company, as an Oklahoma corporation, is empowered by section 1031 of the
Oklahoma General Corporation Act, subject to the procedures and limitations
stated therein, to indemnify any person against expenses (including attorneys'
fees), judgments, fines and amounts paid in settlement actually and reasonably
incurred by him in connection with any threatened, pending or completed action,
suit or proceeding in which such person is made or threatened to be made a
party by reason of his being or having been a director, officer, employee or
agent of the Company. The statute provides that indemnification pursuant to its
provisions is not exclusive of other rights of indemnification to which a
person may be entitled under any bylaw, agreement, vote of shareholders, or
disinterested directors, or otherwise. Article VIII of the By-laws of the
Company provides that directors and officers of the Company shall be
indemnified by the Company to the fullest extent permitted by Oklahoma law as
now or hereafter enforced, including the advance of related expenses. If any
determination is required under applicable law as to whether a director or
officer is entitled to indemnification, such determination shall be made by the
Board, by vote of a quorum of disinterested directors, or by independent legal
counsel by written opinion or by shareholders.
 
   The Certificate of Incorporation of the Company provides that a director of
the corporation shall not be personally liable to the corporation or its
shareholders for monetary damages for breach of fiduciary duty as a director,
except for liability for (1) any breach of the director's duty of loyalty to
the corporation or its shareholders, (2) acts or omissions not in good faith or
which would involve intentional misconduct or a knowing violation of law, (3)
payment of unlawful dividends or unlawful stock purchases or redemptions, or
(4) any transaction from which the director derived an improper personal
benefit.
 
   Pursuant to Article VIII of the By-laws of the Company, upon authorization
and determination (1) by the board of directors by a majority of a quorum
consisting of directors who were not parties to the action, suit, or proceeding
involved; (2) if such a quorum is not obtainable, or even if obtainable and a
quorum of disinterested directors so directs, by independent counsel in a
written opinion; or (3) by the stockholders, the Company is obligated to
indemnify any person who incurs liability by reason of the fact that he is or
was a director, officer, employee, or agent of the Company, or is or was
serving at its request as a director, officer, employee, or agent of another
corporation, partnership, joint venture, trust or other enterprise, or as a
member of any committee or similar body, if he acted in good faith and in a
manner he reasonably believed to be in or not opposed to the best interests of
the Company, and with respect to any criminal action or proceeding, had no
reasonable cause
 
                                      II-1
<PAGE>
 
to believe his conduct was unlawful. However, in an action by or in the right
of the Company, no indemnification will be made if such person shall be
adjudged to be liable to the Company, unless such indemnification is allowed by
a court of competent jurisdiction.
 
   Under an insurance policy obtained by the Company, coverage of Company
officers and directors against liability for neglect, errors, omissions, or
breaches of duty in their capacities as such is provided for both the Company,
to the extent that it is obligated to indemnify such officers and directors,
and the officers and directors themselves. Such coverage is provided in the
amount of $75,000,000, with a retained limit by the Company of $250,000. The
insurance company is obligated to pay any loss in excess of the $250,000
retained limit and defense costs from the first dollar, up to the policy limit
of $75,000,000. Among the policy exclusions are those which exclude coverage
for accounting for profits made within the meaning of Section 16(b) of the
Securities Exchange Act of 1934, claims based upon or attributable to directors
and officers gaining any personal profit or advantage to which such individuals
are not legally entitled, and for any claims brought about or attributable to
the dishonesty of an officer or director.
 
   It is recognized that the above-summarized provisions of the Company's
bylaws and the applicable Oklahoma General Corporation Law may be sufficiently
broad to indemnify officers, directors, and controlling persons of the Company
against liabilities arising under said Act.
 
   The Company and Western Resources, Inc. ("WRI") have entered into a
Registration Rights Agreement which provides for indemnification of the
Company's directors, officers, employees and controlling persons, if any, in
any offering or sale of shares of common stock, obtainable upon conversion of
the Series A Convertible Preferred Stock or Series B Convertible Preferred
Stock, against any claims (including amounts paid in settlement), or actions or
proceedings in respect thereof, arising out of or based upon an untrue
statement or alleged untrue statement of a material fact contained in such
registration statement or prospectus contained therein, or any document
incorporated by reference therein, or arising out of or based upon any omission
or alleged omission to state therein a material fact required to be stated or
necessary to make the statements therein not misleading, in each case only to
the extent that such untrue statement or alleged untrue statement or omission
or alleged omission was made in reliance upon and in conformity with written
information furnished to the Company by WRI or an agent or underwriter thereof
expressly for use therein.
 
 
                                      II-2
<PAGE>
 
Item 16. Exhibits.
 
<TABLE>
<CAPTION>
     Exhibit
     Number    Description
     -------   -----------
     <C>       <S>
      1.1      Form of Underwriting Agreement.
      4.1      Form of Indenture between the Company and Chase Bank of Texas,
               National Association as Trustee, relating to the Securities
               (incorporated by reference to Exhibit 4.1 to the Company's
               registration statement on Form S-3 (Commission File No. 333-
               622279)).
      4.2      Form of First Supplemental Indenture (incorporated by reference
               to Exhibit 5(a) to the Company's current report on Form 8-K
               dated September 24, 1998).
      4.3      Form of Second Supplemental Indenture (incorporated by reference
               to Exhibit 5(b) to the Company's current report on Form 8-K
               dated September 24, 1998).
      4.4      Form of Third Supplemental Indenture (incorporated by reference
               to Exhibit 4 to the Company's current report on Form 8-K dated
               February 8, 1999).
      4.5      Form of Fourth Supplemental Indenture.
      4.6      Form of Debt Security (incorporated by reference to Exhibit 4.2
               to the Company's registration statement on Form S-3 (Commission
               File No. 333-62279)).
      5.1      Opinion of Gable & Gotwals regarding the validity of the Debt
               Securities.
     12.1      Statement of Calculation of Ratio of Earnings to Fixed Charges
               (incorporated by reference to Exhibit 12(a) to the Company's
               current report on Form 8-K dated April 15, 1999).
     23.1      Consent of KPMG LLP.
     23.2      Consent of Arthur Andersen LLP.
     23.3      Consent of Gable & Gotwals (included in Exhibit 5.1).
     24.1      Powers of Attorney (included in the signature page of this
               registration statement).
     25.1      Statement of Eligibility of Chase Bank of Texas, National
               Association under the Trust Indenture Act of 1939 on Form T-1
               relating to the Indenture.
</TABLE>
 
Item 17. Undertakings.
 
   (a) The undersigned registrant hereby undertakes:
 
       (1) To file, during any period in which offers or sales are being
    made, a post-effective amendment to this registration statement:
 
         (i) To include any prospectus required by Section 10(a)(3) of the
      Securities Act of 1933;
 
         (ii) To reflect in the prospectus any facts or events arising
      after the effective date of the registration statement (or the most
      recent post-effective amendment thereof) which, individually or in
      the aggregate, represent a fundamental change in the information set
      forth in the registration statement. Notwithstanding the foregoing,
      any increase or decrease in volume of securities offered (if the
      total dollar value of securities offered would not exceed that which
      was registered) and any deviation from the low or high end of the
      estimated maximum offering range may be reflected in the form of
      prospectus filed with the Commission pursuant to Rule 424(b) if, in
      the aggregate, the changes in volume and price represent no more
      than a 20% change in the maximum aggregate offering price set forth
      in the "Calculation of Registration Fee" table in the effective
      registration statement;
 
         (iii) To include any material information with respect to the
      plan of distribution not previously disclosed in the registration
      statement or any material change to such information in the
      registration statement;
 
 
                                     II-3
<PAGE>
 
  provided, however, that paragraphs (a) (1) (i) and (a) (1) (ii) do not
  apply if the registration statement is on Form S-3, Form S-8 or Form F-3,
  and the information required to be included in a post-effective amendment
  by those paragraphs is contained in periodic reports filed with or
  furnished to the Commission by the registrant pursuant to Section 13 or
  15(d) of the Securities Exchange Act of 1934 that are incorporated by
  reference in the registration statement.
 
     (2) That, for the purpose of determining any liability under the
  Securities Act of 1933, each such post-effective amendment shall be deemed
  to be a new registration statement relating to the securities offered
  therein, and the offering of such securities at that time shall be deemed
  to be the initial bona fide offering thereof.
 
     (3) To remove from registration by means of a post-effective amendment
  any of the securities being registered which remain unsold at the
  termination of the offering.
 
   (b) The undersigned registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
registrant's annual report pursuant to section 13(a) or section 15(d) of the
Securities Exchange Act of 1934 (and, where applicable, each filing of an
employee benefit plan's annual report pursuant to section 15(d) of the
Securities Exchange Act of 1934) that is incorporated by reference in the
registration statement shall be deemed to be a new registration statement
relating to the securities offered therein, and the offering of such securities
at the time shall be deemed to be the initial bona fide offering thereof.
 
   (c) Insofar as indemnification for liabilities arising under the Securities
Act of 1933 may be permitted to directors, officers and controlling persons of
the registrant pursuant to the foregoing provisions, or otherwise, the
registrant has been advised that in the opinion of the Securities and Exchange
Commission such indemnification is against public policy as expressed in the
Act and is, therefore, unenforceable. In the event that a claim for
indemnification against such liabilities (other than the payment by the
registrant of expenses incurred or incurred or paid by a director, officer or
controlling person of the registrant in the successful defense of any action,
suit or proceeding) is asserted by such director, officer or controlling person
in connection with the securities being registered, the registrant will, unless
in the opinion of its counsel the matter has been settled by controlling
precedent, submit to a court of appropriate jurisdiction the question whether
such indemnification by it is against public policy as expressed in the Act and
will be governed by the final adjudication of such issue.
 
   (d) The undersigned registrant hereby undertakes that:
 
     (1) for purposes of determining any liability under the Securities Act
  of 1933, the information omitted from the form of prospectus filed as part
  of this registration statement in reliance upon Rule 430A and contained in
  a form of prospectus filed by the registrant pursuant to Rule 424 (b) (1)
  or (4) or 497(h) under the Securities Act shall be deemed to be part of
  this registration statement as of the time it was declared effective.
 
     (2) For the purpose of determining any liability under the Securities
  Act of 1933, each post-effective amendment that contains a form of
  prospectus shall be deemed to be a new registration statement relating to
  the Securities offered therein, and the offering of such Securities at that
  time shall be deemed to be the initial bona fide offering thereof.
 
                                      II-4
<PAGE>
 
                                   SIGNATURES
 
   Pursuant to the requirements of the Securities Act of 1933, the registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-3 and has duly caused this registration
statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Tulsa, State of Oklahoma, on April 15, 1999.
 
                                          ONEOK, INC.
 
                                                   /s/ James C. Kneale
                                          By: _________________________________
                                                       James C. Kneale
                                               Vice President, Chief Financial
                                                    Officer and Treasurer
 
   KNOW ALL MEN BY THESE PRESENTS, that each person whose signature appears
immediately below constitutes and appoints James C. Kneale and John A.
Gaberino, Jr. his or her true and lawful attorney-in-fact and agent, with full
power of substitution and resubstitution, for him or her and in his or her
name, place and stead, in any and all capacities, to sign any and all
amendments (including post-effective amendments) to this registration
statement, and registration statements filed pursuant to Rule 462 under the
Securities Act of 1933, and to file the same with all exhibits thereto and
other documents in connection therewith, with the Securities and Exchange
Commission, granting unto said attorney-in-fact and agents, and each of them,
full power and authority to do and perform each and every act and thing
requisite and necessary to be done, as fully to all intents and purposes as he
or she might or could do in person, hereby ratifying and confirming all that
said attorney-in-fact and agents or any of them, or their or his substitute or
substitutes, may lawfully do or cause to be done by virtue hereof.
 
   Pursuant to the requirements of the Securities Act of 1933, this
registration statement has been signed below by the following persons in the
capacities and on the dates indicated.
 
<TABLE>
<CAPTION>
             Signature                           Title                    Date
             ---------                           -----                    ----
 
<S>                                  <C>                           <C>
      /s/ Larry W. Brummett          Chairman of the Board, Chief    April 15, 1999
____________________________________  Executive Officer and
         Larry W. Brummett            Director (Principal
                                      Executive Officer)
 
         /s/ W. M. Bell              Director                        April 15, 1999
____________________________________
             W. M. Bell
 
       /s/ D. R. Cummings            Director                        April 15, 1999
____________________________________
           D. R. Cummings
 
      /s/ Barry D. Epperson          Vice President, Controller      April 15, 1999
____________________________________  and Chief Accounting
         Barry D. Epperson            Officer (Principal
                                      Accounting Officer)
 
         /s/ W. L. Ford              Director                        April 15, 1999
____________________________________
             W. L. Ford
 
</TABLE>
 
 
                                      II-5
<PAGE>
 
<TABLE>
<CAPTION>
             Signature                           Title                    Date
             ---------                           -----                    ----
<S>                                  <C>                           <C>
        /s/ H. R. Fricke             Director                        April 15, 1999
____________________________________
            H. R. Fricke
 
         /s/ D. L. Kyle              President, Chief Operating      April 15, 1999
____________________________________  Officer and Director
             D. L. Kyle
 
         /s/ D. T. Lake              Director                        April 15, 1999
____________________________________
             D. T. Lake
 
        /s/ B. H. Mackie             Director                        April 15, 1999
____________________________________
            B. H. Mackie
 
       /s/ James C. Kneale           Vice President, Chief           April 15, 1999
____________________________________  Financial Officer and
          James C. Kneale             Treasurer (Principal
                                      Financial Officer)
 
        /s/ G. D. Parker             Director                        April 15, 1999
____________________________________
            G. D. Parker
 
         /s/ S. L. Young             Director                        April 15, 1999
____________________________________
            S. L. Young
</TABLE>
 
                                      II-6
<PAGE>
 
                                 EXHIBIT INDEX
 
<TABLE>
<CAPTION>
Exhibit
Number   Description
-------  -----------
<S>      <C>
  1.1    Form of Underwriting Agreement.
 
  4.1    Form of Indenture between the Company and Chase Bank of Texas, National
         Association as Trustee, relating to the Securities (incorporated by reference to
         Exhibit 4.1 to the Company's registration statement on Form S-3 (Commission File
         No. 333-62279)).
 
  4.2    Form of First Supplemental Indenture (incorporated by reference to Exhibit 5(a) to
         the Company's current report on Form 8-K dated September 24, 1998).
 
  4.3    Form of Second Supplemental Indenture (incorporated by reference to Exhibit 5(b)
         to the Company's current report on Form 8-K dated September 24, 1998).
 
  4.4    Form of Third Supplemental Indenture (incorporated by reference to Exhibit 4 to
         the Company's current report on Form 8-K dated February 8, 1999).
 
  4.5    Form of Fourth Supplemental Indenture.
 
  4.6    Form of Debt Security (incorporated by reference to Exhibit 4.2 to the Company's
         registration statement on Form S-3 (Commission File No. 333-62279)).
 
  5.1    Opinion of Gable & Gotwals regarding the validity of the Debt Securities.
 
 12.1    Statement of Calculation of Ratio of Earnings to Fixed Charges (incorporated by
         reference to Exhibit 12(a) to the Company's current report on Form 8-K dated April
         15, 1999).
 
 23.1    Consent of KPMG LLP.
 
 23.2    Consent of Arthur Andersen LLP.
 
 23.3    Consent of Gable & Gotwals (included in Exhibit 5.1).
 
 24.1    Powers of Attorney (included in the signature page of this registration
         statement).
 
 25.1    Statement of Eligibility of Chase Bank of Texas, National Association under the
         Trust Indenture Act of 1939 on Form T-1 relating to the Indenture.
</TABLE>
