
<PAGE>   1
                                                     Registration No. 333-______
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AS FILED WITH THE SECURITIES AND EXCHANGE COMMISSION ON JUNE 18, 1999.
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                       SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549
                                    FORM S-8
                             REGISTRATION STATEMENT
                                      UNDER
                           THE SECURITIES ACT OF 1933

                               ------------------


                      ONEOK, Inc. Long-Term Incentive Plan

                               ------------------


                                   ONEOK, INC.

                  100 West Fifth Street, Tulsa, Oklahoma 74103
      (Name of the issuer of the equity securities being offered pursuant
              to the Plan and the address of its principal office)

          OKLAHOMA                                               73-1520922
(State or other jurisdiction of                                (IRS Employer
Incorporation or organization)                               Identification No.)

         JIM KNEALE                                           DONALD A. KIHLE
Vice President, Chief Financial                               Gable & Gotwals
   Officer and Treasurer                                   100 West Fifth Street
         ONEOK, Inc.                                            Suite 1000
   100 West Fifth Street                                   Tulsa, Oklahoma 74103
   Tulsa, Oklahoma 74103                                       (918) 585-8141
      (918) 588-7000

         (Name, addresses, and telephone numbers of agents for service)

                               ------------------


Appropriate date of commencement of proposed sale pursuant to the Plan: from
                  time to time after the effective date hereof

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                         CALCULATION OF REGISTRATION FEE

--------------------------------------------------------------------------------
<TABLE>
<CAPTION>

                                                     Proposed          Proposed
                                                     Maximum           Maximum
                                                     Offering          Aggregate                 Amount of
Title of Securities        Amount to be              Price             Offering                  Registration
to be Registered           Registered(1)             Per Unit(2)       Price(3)                  Fee


--------------------------------------------------------------------------------------------------------------

--------------------------------------------------------------------------------------------------------------
<S>                        <C>                       <C>               <C>                       <C>
Common stock,
$0.01 par value            1,445,200                 $ 29.96875        $ 43,310,837              $ 12,040

--------------------------------------------------------------------------------------------------------------
</TABLE>

Exhibits Index on Page 5.



<PAGE>   2

(1)      Represents the estimated maximum amount of common stock of ONEOK, Inc.
         (hereinafter referred to as "Common Stock") which could be acquired
         under the ONEOK, Inc. Long-Term Incentive Plan (hereinafter referred to
         as the "Plan") either directly from ONEOK, Inc. (hereinafter referred
         to as the "Company"), or from purchases in the open market during the
         years of operation of the Plan.

(2)      Based on price of $ 29.96875 per share of the Common Stock, the average
         sales price of the Common Stock published in the Wall Street Journal
         reports of the New York Stock Exchange Composite Transaction for June
         14, 1999.

(3)      Estimated pursuant to Rule 457(c) solely for the purpose of calculating
         the registration fee.





                                       2



<PAGE>   3




                                     PART I
              INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

The document(s) containing the information specified in Part I of Form S-8 will
be sent or given to employees as specified in Rule 428(b)(1). These documents
(and the documents incorporated by reference pursuant to Item 3 of Part II of
this Registration Statement) taken together, constitute the prospectus for
purpose of Section 10(a) of the Securities Act of 1933, as amended.


                                     PART II
               INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

                                     ITEM 3
                     INCORPORATION OF DOCUMENTS BY REFERENCE

The "Company", and the Plan hereby incorporate by reference in this Registration
Statement the following documents of the Company (SEC File No. 1-2572)
heretofore filed with the Securities and Exchange Commission:

         (1)      Annual Report on Form 10-K of ONEOK, Inc. for the year ended
                  August 31, 1998.

         (2)      All other reports filed by ONEOK or the Plan pursuant to
                  Section 13(a) or 15(d) of the Securities Exchange Act of 1934
                  since August 31, 1998. Form 8-K filed September 24, 1998. Form
                  8-K/A filed October 2, 1999. Form 8-K filed October 21, 1998.
                  Form 8-K filed December 16, 1998. Form 8-K filed December 16,
                  1998. Form 10-Q filed January 13, 1999. Form 8-K filed January
                  25, 1999. Form 8-K/A filed January 26, 1999. Form 8-K filed
                  January 25, 1999. Form 8-K filed February 8, 1999. Form 8-K
                  filed February 16, 1999. Form 8-K filed February 23, 1999.
                  Form 8-K filed February 24, 1999. Form 8-K filed February 24,
                  1999. Form 8-K filed February 26, 1999. Form 8-K filed March
                  5, 1999. Form 8-K filed March 19, 1999. Form 10-Q filed April
                  14, 1999. Form 8-K filed April 15, 1999. Form 8-K/A filed
                  April 29, 1999. Form 8-K filed April 19, 1999. Form 8-K filed
                  April 22, 1999. Form 8-K filed April 26, 1999. Form 8-K filed
                  May 3, 1999. Form 8-K filed May 13, 1999.

         (3)      ONEOK's Proxy Statement dated December 16, 1998 in connection
                  with its Annual Meeting of Shareholders held January 21, 1999.

         (4)      In addition, there is incorporated herein by reference all
                  documents filed subsequent to the date hereof, by the Company
                  and the Plan pursuant to Sections 13(a), 13(c), 14, or 15(d)
                  of the Securities Exchange Act of 1934, as amended, prior to
                  the filing of a post-effective amendment which indicates that
                  all securities have been sold or which deregisters all
                  securities then remaining unsold. Such documents are deemed to
                  be a part hereof from the date of filing of such documents.

                                     ITEM 4
                            DESCRIPTION OF SECURITIES

The following is a description of the Company's Common Stock offered.

The holders of the Company's Common Stock are entitled to receive such dividends
as may be declared by the Board of Directors out of funds legally available
therefor. Each outstanding share of Common Stock is entitled to full voting
rights for the election of directors and for all other purposes with one vote
for each share of Common Stock. In the event of any liquidation, dissolution, or
winding up of the Company, or any reduction of its capital resulting in any
distribution of its assets to its stockholders, The holders of Common Stock
shall be entitled to receive, pro rata, all the remaining assets of the Company
available for distribution to its stockholders. Each share

                                        3

<PAGE>   4




of Common Stock includes an associated right, each right ("Right") entitling the
holder to purchase one one-hundredth of a share of Series C Participating
Preferred Stock, par value $0.01 per share of the Company pursuant to a Rights
Agreement between the Company and a designated rights agent (the "Rights
Agreement"). The designation of Rights (Exhibit 4(c) hereto) is incorporated
herein by reference.

                                     ITEM 5
                     INTERESTS OF NAMED EXPERTS AND COUNSEL


The legality of the securities which may be purchased under the Plan has been
passed upon by the firm of Gable & Gotwals, 100 West Fifth Street, Suite 1000,
Tulsa, Oklahoma 74103, counsel for the Company. The firm of Gable & Gotwals, has
reviewed the statements made as to matters of law and legal conclusions under
"Securities to be Offered" and such statements are set forth in the documents
which form a part of the prospectus in reliance upon its authority as an expert.

                                     ITEM 6
                    INDEMNIFICATION OF DIRECTORS AND OFFICERS

Pursuant to Article VIII of the bylaws of ONEOK, Inc. upon authorization and
determination either (1) by the board of directors by a majority of a quorum is
not 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 interest of
the Company, and with respect to any criminal action or proceeding, had no
reasonable cause 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 insurance obtained by the Company, coverage of Company officers and
directors against liability for neglect, errors, omission, 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 companies are
obligated to pay covered losses in excess of the $250,000 retained limit, up to
the policies' limits of $75,000,000. Among the policies' exclusions are those
which exclude coverage for accounting for profits made within the meaning of
Section 16(b) of the Securities 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.

The registrant has been advised that, in the opinion of the Securities and
Exchange Commission, provisions providing for the indemnification by the
corporation of its officers, directors, and controlling persons against
liabilities imposed by the Securities Act of 1933 are against public policy as
expressed in said Act and are therefore unenforceable. It is recognized that the
above-summarized provisions of the registrant's bylaws and the applicable
Oklahoma General Corporation Law may be sufficiently broad to indemnify
officers, directors, and controlling persons of the registrant against
liabilities arising under said Act. Therefore, in the event that a claim of
indemnification against liability under said Act (other than the payment by the
registrant of expenses incurred or paid by a director, officer, or controlling
person of the registrant in the successful defense of any action, suit, or
proceeding) shall be asserted by an officer, director, or controlling person
under said provisions, the registrant will, unless in the opinion of its counsel
the question has already been settled by the controlling precedent, submit to a
court of appropriate jurisdiction the question of whether or not such
indemnification by it is against public policy as expressed in said Act and will
be governed by the final adjudication of such issue.

                                        4

<PAGE>   5




                                     ITEM 8
                                    EXHIBITS

The following exhibits are attached hereto or incorporated by reference herein:

<TABLE>
<CAPTION>

                                                                                        Page Number or
                                                                                        Incorporation by
                                                                                        Reference to
                                                                                        -----------------

<S>                                                                             <C>
(4)(a)   Article "Fourth" of the Certificate of Incorporation                   Incorporated by reference
          of ONEOK, Inc. (Preferred Stock and Common Stock)                     from Exhibit 3.1 to
                                                                                Amendment No. 3 to Registration
                                                                                Statement on Form S-4 filed
                                                                                August 31, 1997


(4)(b)   Certificate of Designation for Convertible Preferred                   Incorporated by reference
         stock of WAI, Inc. (Now ONEOK, Inc.) filed November 26, 1997           from Exhibit 3.3 to
                                                                                Amendment No. 3 to Registration
                                                                                Statement on Form S-4 filed
                                                                                August 31, 1997


(4)(c)   Certificate of Designation for Series C Participating                  Incorporated by reference
         Preferred Stock of ONEOK, Inc., filed November 26, 1998                from Exhibit No. 1 to
                                                                                Form 8-A, filed November 26,
                                                                                1997

(5)      Opinion of Gable & Gotwals, Inc.                                                 11


(23)(a)  Consent of Gable & Gotwals, Inc.
         (See Item 5)

(23)(b)  Consent of KPMG LLP                                                              13

(24)     Powers of Attorney (Included on pages 8 and 9)

(99)     ONEOK, Inc. Long-Term Incentive Plan                                             14
</TABLE>














                                        5

<PAGE>   6




                                     ITEM 9
                                  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:

                  (a)      To include any prospectus required by Section 10
                           (a)(3) of the Securities Act of 1933;

                  (b)      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;

                  (c)      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.

         (2)      That, for the purpose of determining any liability under the
                  Securities Act of 1933, each 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 the time of shall be deemed to be the initial
                  bona fide offering thereof.

         (3)      To remove from registration by means of 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 purpose 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
         herein, and the offering of such securities at that time shall be
         deemed to be the initial bona fide offering thereof.

c.       The undersigned registrant hereby undertakes to deliver or cause to be
         delivered with the prospectus to each employee to whom the prospectus
         is sent or given, the latest Annual Report to Shareholders unless such
         employee otherwise has received a copy of such report, in which case
         the registrant shall state in the prospectus that it will promptly
         furnish, without charge, a copy of such report on written request of
         the employee. If the last fiscal year of the registrant has ended
         within 120 days prior to the use of the prospectus, the Annual Report
         of the registrant of the preceding fiscal year may be delivered, but
         within such 120-day period the Annual Report for the last fiscal year
         will be furnished to each such employee.

         The undersigned registrant hereby undertakes to transmit or cause to be
         transmitted to all employees participating in the Plan who do not
         otherwise receive such material as stockholders or the registrant, at
         the time and in the manner such material is sent to its stockholders,
         copies of all reports, proxy statements, and other communications
         distributed to its stockholders generally.

d.       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

                                        6

<PAGE>   7




         the event that a claim for indemnification against such liabilities
         (other than the payment by the registrant of expenses 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.










                                        7

<PAGE>   8




                                    SIGNATURE

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-8, and has duly caused this registration
statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Tulsa and the State of Oklahoma, on the 17th day of
June, 1999.


                                           ONEOK, Inc.


                                           By: Larry Brummett
                                               ---------------------------------
                                               Larry Brummett, Chairman of the
                                               Board and Chief Executive
                                               Officer












                                        8

<PAGE>   9




                                POWER OF ATTORNEY

Each person whose individual signature appears below hereby authorizes Larry
Brummett and Jim Kneale, or either of them, as attorney-in-fact with full power
of substitution, to execute in the name and on behalf of each person,
individually and in each capacity stated below, and to file any and all
amendments to this registration statement, including any and all post-effective
amendments and all instruments necessary or incidental in connection therewith.


                                   SIGNATURES

Pursuant to the requirements of the Securities Act of 1933, the undersigned
certify that to the best of their knowledge and belief, the registrant meets all
the requirements for filing on Form S-8. This registration statement has been
signed below by the following persons in the capacities indicated in the City of
Tulsa and the State of Oklahoma, on this 17th day of June, 1999.


Larry W. Brummett                          Jim Kneale
-------------------------------            ---------------------------------
Larry W. Brummett                          Jim Kneale
Chairman of the Board,                     Vice President,
Chief Executive                            Chief Financial Officer, and
Officer, and Director                      Treasurer

David L. Kyle                              Barry D. Epperson
-------------------------------            ---------------------------------
David L. Kyle                              Barry D. Epperson
President, Chief Operating                 Vice President, Controller, and
Officer, and Director                      Chief Accounting Officer

Edwyna G. Anderson                         Douglas T. Lake
-------------------------------            ---------------------------------
Edwyna G. Anderson                         Douglas T. Lake
Director                                   Director

William M. Bell                            Bert H. Mackie
-------------------------------            ---------------------------------
William M. Bell                            Bert H. Mackie
Director                                   Director

Douglas R. Cummings
-------------------------------            ---------------------------------
Douglas R. Cummings                        Douglas Ann Newsom
Director                                   Director

William L. Ford                            Gary D. Parker
-------------------------------            ---------------------------------
William L. Ford                            Gary D. Parker
Director                                   Director

Howard R. Fricke
-------------------------------            ---------------------------------
Howard R. Fricke                           J. D. Scott
Director                                   Director

                                           Stanton L. Young
                                           ---------------------------------
                                           Stanton L. Young
                                           Director


                                        9

<PAGE>   10




                                  EXHIBIT INDEX

<TABLE>
<CAPTION>

                                                                                        PAGE NUMBER OR
EXHIBIT                                                                                 INCORPORATION BY
NUMBER            DESCRIPTION                                                           REFERENCE TO
-------           -----------                                                           ----------------

<S>               <C>                                                                   <C>
(4)(a)            Article "Fourth" of the Certificate of Incorporation                  Incorporated by reference
                  of ONEOK, Inc. (Preferred Stock and Common Stock)                     from Exhibit 3.1 to
                                                                                        Amendment No. 3 to
                                                                                        Registration Statement on
                                                                                        Form S-4 filed August 31,
                                                                                        1997


(4)(b)            Certificate of Designation for Convertible Preferred                  Incorporated by reference
                  stock of WAI, Inc. (Now ONEOK, Inc.) filed November 26, 1997          from Exhibit 3.3 to
                                                                                        Amendment No. 3 to
                                                                                        Registration Statement on
                                                                                        Form S-4 filed August 31,
                                                                                        1997


(4)(c)            Certificate of Designation for Series C Participating                 Incorporated by reference
                  Preferred Stock of ONEOK, Inc., filed November 26, 1998               from Exhibit No. 1 to
                                                                                        Form 8-A, filed November
                                                                                        26, 1997


(5)               Opinion of Gable & Gotwals                                            11

(23)(a)           Consent of Gable & Gotwals (See Item 5)

(23)(b)           Consent of KPMG LLP                                                   13

(24)              Powers of Attorney (Included on pages 8 and 9)

(99)              ONEOK, Inc. Long-Term Incentive Plan                                  14
</TABLE>


                                       10
