
<PAGE>   1
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549



                                    FORM 8-K


                                 CURRENT REPORT
                     PURSUANT TO SECTION 13 OR 15(d) OF THE
                         SECURITIES EXCHANGE ACT OF 1934



                                January 25, 1999
                        (Date of earliest event reported)



                                   ONEOK, Inc.
             (Exact name of registrant as specified in its charter)



         Oklahoma                      1-2572                 73-1520922
(State or other jurisdiction        (Commission             (IRS Employer
       of incorporation)            File Number)         Identification No.)


                        100 West Fifth Street; Tulsa, OK
                    (Address of principal executive offices)


                                      74103
                                   (Zip code)


                                 (918) 588-7000
              (Registrant's telephone number, including area code)


                                 Not Applicable
          (Former name or former address, if changed since last report)


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Items 1-4                           Not applicable.

Item 5                              Other Events:

                                    Pursuant to the terms and conditions of a
                                    certain Agreement and Plan of Merger, dated
                                    December 14, 1998, among ONEOK, Inc., an
                                    Oklahoma corporation ("ONEOK"), Oasis
                                    Acquisition Corporation, a California
                                    corporation and newly formed subsidiary of
                                    ONEOK ("OAC"), and Southwest Gas
                                    Corporation, a California corporation
                                    ("Southwest"), Southwest will be merged into
                                    OAC with OAC as the surviving corporation.
                                    Immediately after that, OAC will be merged
                                    into ONEOK with ONEOK as the surviving
                                    corporation. The mergers are subject to
                                    approval by Southwest's shareholders, state
                                    regulatory agencies, and typical closing
                                    conditions. It is anticipated that the
                                    transactions can be closed later in 1999.

                                    In order for ONEOK to issue certain
                                    securities during the interim period, ONEOK
                                    must comply with Rule 3-05 and Rule
                                    11-01(a)(8) of Regulation S-X of the
                                    Securities and Exchange Commission with
                                    respect to disclosure of certain financial
                                    statements of the business acquired or to be
                                    acquired and pro forma financial information
                                    relating to the probable merger
                                    transactions. Therefore, ONEOK is filing as
                                    exhibits hereto certain financial
                                    information relating to Southwest and
                                    required pro forma financial information.

Item 6                              Not applicable.

Item 7                              Financial Statements, Pro Forma Financial
                                    Information and Exhibits


Exhibit No.:                        Description.

12                                  Included herein - Computation of Ratio of
                                    Earnings to Combined Fixed Charges and
                                    Preferred Stock Dividend Requirements

12.a                                Included herein - Computation of Ratio of
                                    Earnings to Fixed Charges

23.a                                Consent of Arthur Andersen LLP

99.a                                Included herein - Unaudited Pro Forma 
                                    Combined Condensed Financial Statements

99.b                                Incorporated by Reference - Southwest Gas
                                    Corporation Form 10-K for the year ended
                                    December 31, 1997, filed March 31, 1998

99.c                                Incorporated by Reference - Southwest Gas
                                    Corporation Form 10-Q for the Quarter ended
                                    March 31, 1998, filed May 6, 1998

99.d                                Incorporated by Reference - Southwest Gas
                                    Corporation Form 10-Q for the Quarter ended
                                    June 30, 1998, filed August 14, 1998

99.e                                Incorporated by Reference - Southwest Gas
                                    Corporation Form 10-Q for the Quarter Ended
                                    September 30, 1998, filed November 16, 1998



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99.f                                Incorporated by Reference - Reports on Form
                                    8-K filed by Southwest Gas Corporation

                                    8-K filed February 11, 1998 
                                    8-K filed April 15, 1998 
                                    8-K filed April 29, 1998 
                                    8-K filed July 17, 1998 
                                    8-K filed July 17, 1998 
                                    8-K filed July 27, 1998 
                                    8-K filed August 7, 1998
                                    8-K filed September 25, 1998 
                                    8-K filed October 29, 1998 
                                    8-K filed December 14, 1998

Item 8                              Not applicable.



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                                   SIGNATURE



     Pursuant to the requirements of the Securities Exchange Act of 1934, the
registrant has duly caused this report to be signed on its behalf by the
undersigned hereunto duly authorized, on this 25th of January, 1999.


                                             ONEOK, Inc.

                                        By:  /s/ JERRY D. NEAL
                                             ----------------------------------
                                             Vice President, Chief Financial
                                             Officer, and Treasurer

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                               INDEX TO EXHIBITS



<TABLE>
<CAPTION>
EXHIBIT
NUMBER                             DESCRIPTION
-------                            -----------
<S>            <C>
 12             Included herein - Computation of Ratio of Earnings to Combined 
                Fixed Charges and Preferred Stock Dividend Requirements
                 
 12.a           Included herein - Computation of Ratio of Earnings to  Fixed
                Charges
                
 23.a           Consent of Arthur Andersen LLP

 99.a           Unaudited Pro Forma Combined Condensed Financial Statements
</TABLE>

