If that particular scenario develops, the nod goes to the team ranked highest in the first BCS poll following the completion of the Big 12 regular season.
So, if Oklahoma beats Texas Tech, it'll likely eliminate the Red Raiders from getting a trip to the Big 12 championship game. Texas would likely be the highest ranked team at that point...but, if Oklahoma is impressive enough in their victory over Tech, they may somehow jump the Longhorns. It's gonna be interesting.