I think you are wrong. An undefeated Big 12 champ that ends the season beating Baylor and Oklahoma gets in. They just do. Then they get rocked by Alabama. But they make the playoff.
Oklahoma vs. ND is an interesting debate. As someone said a few pages back, best thing for us is Baylor beating Okie State - ideally by a lot - this week and knocking them down a peg before they play the Sooners. Also TCU losing two more games. The Big 12 looks a lot less rugged with TCU unranked and Okie State down around 15-20.
How about the Baylor scenario? Baylor wins out (Oklahoma State, TCU, Texas). Oklahoma State beats OU, giving them their second loss. Baylor has the head to head over Oklahoma State. Does a one loss Baylor get in over a one loss ND?