
<PAGE>   1

                                                                   EXHIBIT 99.a

===============================================================================
       ONEOK FINANCIAL NEWS
===============================================================================

ONEOK, INC.                                 CONTACT: WELDON WATSON, 918/588-7158
P. O. BOX 871
TULSA, OK 74102-0871                      FOR IMMEDIATE RELEASE, AUGUST 10, 1999


                           ONEOK/SOUTHWEST GAS MERGER
                            APPROVED BY SHAREHOLDERS



         TULSA, Oklahoma - The merger of ONEOK, Inc., (NYSE:OKE) and Southwest
Gas Corporation (NYSE:SWX) was approved at the annual meeting of Southwest Gas
shareholders today in Las Vegas, Nevada. Shareholders approved a cash offer of
$30 per share of Southwest Gas common stock for a total of $918 million. The
total transaction including assumed debt is estimated at $1.8 billion.
Approximately 79 percent of the shares voted at the meeting were in support of
the merger.

         Larry Brummett, chairman and chief executive officer of the company,
said, "We are very pleased with the support of the Southwest shareholders and
will continue to move as expeditiously as possible to conclude state regulatory
approvals in Arizona and California. We want to conclude this transaction
promptly and successfully in the best interest of shareholders, customers and
employees. We look forward to combining the best practices of all our
distribution operations to better serve our customers in all the states where
we provide gas service."

         The Public Utilities Commission of Nevada unanimously approved the
ONEOK/Southwest Gas merger on June 21. On June 20, ONEOK and Southwest Gas
reached a stipulated agreement on the merger with the Arizona Corporation
Commission staff and the Residential Utility Consumers Office. A hearing is set
before an administrative law judge in Arizona on September 1. Final approval by
the Arizona Commission is expected later in September.

         ONEOK, Southwest Gas and the Office of Ratepayers Advocates of the
California Public Utilities Commission signed an agreement during a settlement
conference in San Francisco on July 30. An administrative law judge is expected
to present the settlement to the California Commission for consideration in
September. The necessary premerger filings with the Justice Department and the
Federal Trade Commission were made early in the process and clearance under the
Hart-Scott-Rodino Act has also been achieved.






<PAGE>   2


         The merger will create the largest stand-alone gas distribution
company in the United States serving 2.6 million customers. ONEOK currently is
the major gas distributor in Kansas and Oklahoma operating as Kansas Gas
Service Company and Oklahoma Natural Gas Company serving 1.4 million customers.
Southwest Gas serves 1.2 million customers as the major gas distributor in
Arizona and Nevada with 100,000 customers in California.

         The terms of the transaction call for three Southwest Gas board
members to join ONEOK's board, filling a current vacancy and two positions that
were vacated due to retirements. Southwest Gas will operate as a division of
ONEOK, Inc., and will retain its name in the markets it serves.

         ONEOK, Inc., is an integrated natural gas company involved in
production, processing, gathering, storage and transmission. The company is
also the largest natural gas distributor in Kansas and Oklahoma.

                                     # # #

STATEMENTS CONTAINED IN THIS RELEASE THAT INCLUDE COMPANY EXPECTATIONS OR
PREDICTIONS AS TO THE ACCRETIVE NATURE OF THE TRANSACTION AND ITS EXPECTED
CONSUMMATION ARE FORWARD-LOOKING STATEMENTS INTENDED TO BE COVERED BY THE SAFE
HARBOR PROVISIONS OF THE SECURITIES ACT OF 1933 AND THE SECURITIES EXCHANGE ACT
OF 1934. IT IS IMPORTANT TO NOTE THAT THE ACTUAL RESULTS COULD DIFFER
MATERIALLY FROM THOSE PROJECTED IN SUCH FORWARD-LOOKING STATEMENTS. AMONG THE
IMPORTANT FACTORS THAT COULD AFFECT ACTUAL RESULTS ARE THE TIMING AND TERMS OF
REGULATORY APPROVALS AND OPERATING RESULTS OF ONEOK AND SOUTHWEST GAS. MORE
INFORMATION ABOUT THE ONEOK AND SOUTHWEST GAS TRANSACTION CAN BE FOUND UNDER
COMPANY NEWS ON THE ONEOK WEB SITE AT www.oneok.com. SERVICE AREA MAPS AND
LOGOS ARE AVAILABLE UNDER MEDIA KIT.
