
<PAGE>   1

                                                                 EXHIBIT (12)(a)

                                   ONEOK, Inc.
                Computation of Ratio of Earnings to Fixed Charges


<TABLE>
<CAPTION>
                                            Six Months                          Years Ended August 31,
                                              Ended          ---------------------------------------------------------------
                                           Feb. 28, 1999        1998         1997         1996         1995         1994
                                           -------------        ----         ----         ----         ----         ----

<S>                                         <C>              <C>          <C>          <C>          <C>          <C> 
Fixed charges, as defined
    Interest on long-term debt              $  17,354        $  30,846    $  31,354    $  31,748    $  32,345    $  32,979
    Other interest                              5,506            3,723        3,376        3,184        4,934        1,855
    Amortization of debt issue costs              504              506          518          530          512          525
    Interest on lease agreements                1,162            2,325        2,266        2,266        2,266        2,266
                                            ---------        ---------    ---------    ---------    ---------    ---------
         Total fixed charges                   24,526           37,400       37,514       37,728       40,057       37,625
                                                                                                                          
Earnings before income taxes                $ 136,765        $ 168,380    $  94,107    $  85,873    $  68,146    $  57,276
                                            ---------        ---------    ---------    ---------    ---------    ---------
                                                                                                                          
Earnings available for fixed charges        $ 161,291        $ 205,780    $ 131,621    $ 123,601    $ 108,203    $  94,901
                                            =========        =========    =========    =========    =========    =========

Ratio of earnings to fixed charges              6.58X            5.50X        3.51X        3.28X        2.70X        2.52X
                                            =========        =========    =========    =========    =========    =========
</TABLE>


           Pro Forma Computation of Ratio of Earnings to Fixed Charges
            Giving Effect to the Proposed Merger as Discussed Herein


<TABLE>
<CAPTION>
                                                 Six Months         Year Ended
                                                   Ended            August 31,
                                               Feb. 28, 1999           1,998
                                               -------------        -----------

<S>                                              <C>                <C>
Fixed charges, as defined
     Interest on long-term debt                  $  70,697          $ 139,252
     Other interest                                  5,506              3,723
     Amortization of debt issue costs                  889              1,275
     Preferred securities distributions                                      
          of subsidiary                             12,112             24,225
     Interest on lease agreements                    1,162              2,325
                                                 ---------          ---------
          Total fixed charges                       90,366            170,800
                                                                             
Earnings before income taxes                     $ 129,180          $ 155,533
                                                 ---------          ---------
                                                                             
Earnings available for fixed charges             $ 219,546          $ 326,333
                                                 =========          =========
                                                                             
Ratio of earnings to fixed charges (a)               2.43X              1.91X
                                                 =========          =========
</TABLE>


For purposes of computing the ratio of earnings to combined fixed charges and
preferred dividend requirements, "earnings" consists of net income plus fixed
charges and income taxes. "Fixed charges" consists of interest charges, the
amortization of debt issue costs, preferred securities distributions of
subsidiary, and the representative interest portion of operating leases.


