If Oklahoma beats Oklahoma State, and OSU has a chance to win that game, then Oklahoma should go to the Big 12 championship game.
Those who say that Texas should go because the Longhorns beat Oklahoma miss the issue of three teams beating each other. So, Texas should go to the Big 12 championship game because they beat the Sooners. No. If that is the case, Texas Tech should go the championship game because they beat the Longhorns. No. Because if head-to-head is the defining criteria, then Oklahoma should go because they beat the Red Raiders. But, head-to-head means that Texas should go because they beat the Sooners. And, this loop plays eternally. So, we have to throw out head-to-head, since Team A beat Team B but lost to Team C and so forth, making head-to-head comparisons useless.
Here's how I see it...
Texas beat Oklahoma and, in turn, was edged by Texas Tech. Texas Tech needed a late touchdown to down Texas and was blasted by Oklahoma. Oklahoma destroyed Texas Tech and was solidly defeated by Texas. Because Oklahoma has the biggest win, of the three teams, they should go to the Big 12 championship team, if they beat OSU.
And, if the Sooners lose to OSU, Texas Tech goes because of their victory over Texas....