
<PAGE>   1

                                                                    EXHIBIT (12)

                                   ONEOK, Inc.
           Computation of Ratio of Earnings to Combined Fixed Charges
                    and Preferred Stock Dividend Requirements


<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
                                                                                                                              
Preferred dividend requirements                     30,570          44,228          285          428          428          428
                                                 ---------       ---------    ---------    ---------    ---------    ---------
                                                                                                                              
Total fixed charges and preferred                                                                                             
     dividend requirements                       $  55,096       $  81,628    $  37,799    $  38,156    $  40,485    $  38,053
                                                 =========       =========    =========    =========    =========    =========
                                                                                                                              
                                                                                                                              
Earnings before income taxes                     $ 136,765       $ 168,380    $  94,107    $  85,873    $  68,146    $  57,276
                                                                                                                              
Total fixed charges                                 24,526          37,400       37,514       37,728       40,057       37,625
                                                 ---------       ---------    ---------    ---------    ---------    ---------
                                                                                                                              
Earnings available for combined                                                                                               
     fixed charges and preferred                                                                                              
     dividend requirements                       $ 161,291       $ 205,780    $ 131,621    $ 123,601    $ 108,203    $  94,901
                                                 =========       =========    =========    =========    =========    =========
                                                                                                                              
Ratio of earnings to combined                                                                                                 
     fixed charges and preferred                                                                                              
     dividend requirements                           2.93X           2.52X        3.48X        3.24X        2.67X        2.49X
                                                 =========       =========    =========    =========    =========    =========
</TABLE>


      Pro Forma Computation of Ratio of Earnings to Combined Fixed Charges
         and Preferred Stock Dividend Requirements Giving Effect to the
                       Proposed Merger as Discussed Herein


<TABLE>
<CAPTION>
                                                 Six Months     Year Ended
                                                   Ended         August 31,
                                               Feb. 28, 1999       1998
                                               -------------    -----------

<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
                                                                          
Preferred dividend requirements                     30,570          44,228
                                                 ---------       ---------
                                                                          
Total fixed charges and preferred                                         
     dividend requirements                       $ 120,936       $ 215,028
                                                 =========       =========
                                                                          
Earnings before income taxes                     $ 129,180       $ 155,533
                                                                          
Total fixed charges                                 90,366         170,800
                                                 ---------       ---------
                                                                          
Earnings available for combined                                           
     fixed charges and preferred                                          
     dividend requirements                       $ 219,546       $ 326,333
                                                 =========       =========
                                                                          
Ratio of earnings to combined                                             
     fixed charges and preferred                                          
     dividend requirements                           1.82X           1.52X
                                                 =========       =========
</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.
"Preferred dividend requirements" consists of the pre-tax preferred dividend
requirement.

