
<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



                                 June 22, 1999
                Date of Report (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.

                  ONEOK, Inc. announced the Public Utilities Commission of
Nevada has approved the proposed merger between ONEOK, Inc., and Southwest Gas
Corporation.
                  On June 22, 1999, the Company issued a press release, a copy
of which is attached hereto as exhibit 99.a and incorporated herein by
reference.

Item 6.           Not Applicable

Item 7.           Financial Statements, Pro Forma Financial Information and
Exhibits.

Exhibit
No.               Description

99.a              Press release issued by ONEOK, Inc. dated June 22, 1999.

Items 8-9.        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 22nd day of June, 1999.


                                          ONEOK, Inc.

                                     By:  Jim Kneale
                                          --------------------------------------
                                          Jim Kneale, Vice President,
                                          Chief Financial Officer, and Treasurer


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                               INDEX TO EXHIBITS



<TABLE>
<CAPTION>
EXHIBIT
NUMBER                             DESCRIPTION
-------                            -----------
<S>                    <C>
 99.a                  Press release issued by ONEOK, Inc. dated June 22, 1999.
</TABLE>
