
<PAGE>   1
                                                                       EXHIBIT 5

             [GABLE GOTWALS MOCK SCHWABE KIHLE GABERINO LETTERHEAD]


                               November 26, 1997



ONEOK, Inc.
100 West Fifth Street
Tulsa, Oklahoma 74103

                                        Re:  S-8 Registration Statement Under
                                             the Securities Act of 1933,
                                             Relating to the Shares of Common
                                             Stock of ONEOK, Inc. in Relation to
                                             the Thrift Plan for Employees of
                                             ONEOK, Inc. and Subsidiaries and
                                             Interests of the Participants in
                                             the Plan

Gentlemen:

     We understand that ONEOK, Inc., an Oklahoma corporation (hereinafter
referred to as the "Company"), will file with the Securities and Exchange
Commission under the Securities Act of 1933, as amended, a Form S-8
Registration Statement relating to the registration of Plan interests, and
registration of shares of the Company's Common Stock, $0.01 par value, (the
"Shares") with respect to the Thrift Plan for Employees of ONEOK, Inc. and
Subsidiaries (the "Plan").

     We have examined (a) the above-mentioned Registration Statement which will
be filed with the Securities and Exchange Commission; (b) the Certificate of
Incorporation and Bylaws of the Company, as amended; (c) the Thrift Plan for
Employees of ONEOK, Inc. and Subsidiaries and the corporate actions taken by
the Board of Directors in connection with the Registration Statement and
related matters; and (d) such other corporate records, certificates of public
officials and officers of the Company and other documents as we have considered
relevant to the matters covered by this opinion.

     In connection with the foregoing, as counsel for the Company, we wish to
advise you as follows:

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ONEOK, Inc.
November 26, 1997
Page 2

     1.   The Company is a corporation validly organized and existing under the
laws of the State of Oklahoma and is duly qualified to do business as a
corporation in the State of Oklahoma.

     2.   The filing of the above-mentioned Registration Statement has been
duly authorized by the proper corporate action on the part of the Company.

     3.   Assuming the Shares are being issued in compliance with the terms and
conditions of the Plan, when the certificates for the Shares have been executed
by the proper officer of the Company, countersigned by the Transfer Agent and
registered by the Registrar thereof, the certificates for such Shares will
represent, and the Shares will constitute, duly authorized, legally issued,
fully paid, non-assessable, valid and legal shares of the Common Stock of the
Company.

     4.   When an employee of the Company becomes a participant in the Plan,
such employee is thereby entitled to an interest in the Plan according to the
provisions of the Plan and the elections made by the participant from time to
time. Such interests in the Plan, when created in accordance with the
provisions of the Plan, will constitute legally issued, fully paid and
non-assessable interests except as may be set forth in the Plan.

     The statements of law and legal conclusions made in the documents
constituting the prospectus furnished in connection with the Registration
Statement pertaining to the Employee Retirement Income Security Act of 1974, as
amended ("ERISA"), have been reviewed by us and are correct. Where appropriate,
the Plan described in the documents constituting the prospectus has been
amended to comply with ERISA, and in such manner that such Plan, as
necessary, may be qualified under the provisions of the Internal Revenue Code of
1986, as amended ("Code"). The Plan has been qualified under the Code. A copy
of the favorable determination letter from the Internal Revenue Service ("IRS")
dated June 7, 1993, concerning the Plan has been considered. Since that
determination, amendments to that Plan have been made to restate the Plan to
conform to the requirements of ERISA and to the extent required by the
provisions of the Code and applicable regulations of the IRS affecting its
qualification. Subject only to the foregoing, it is our opinion that the
written documents comprising the Plan comply with ERISA and the Code.

     We hereby consent to:

     1.   Being named in the Form S-8 Registration Statement and documents
constituting the prospectus which is being furnished, and in any amendments
thereto, as counsel for the Company, passing on legal matters in connection
with the issuance of the Common Stock to the Trustee under the Plan;
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ONEOK, Inc.
November 26, 1997
Page 3


     2.   The making in the Form S-8 Registration Statement and documents
constituting the prospectus, and in any amendments thereto, of the statements
now appearing therein in the caption "interests of Named Experts and Counsel,"
insofar as they are applicable to us; and

     3.   The filing of this opinion as an exhibit to the above-mentioned Form
S-8 Registration Statement.

                                      Very truly yours,

                                      GABLE GOTWALS MOCK SCHWABE KIHLE GABERINO

                                      By  /s/ DONALD A. KIHLE
                                          -------------------------------------
                                          Donald A. Kihle

