Depends on the scenario:
If you think its likely the big 12 only gets one no matter what, you just want to increase the chances ISU is in the championship game. So you want top teams to lose so we have a better chance of being in and then just have to go win that game. So you'd want OSU to win.
If you think there's a chance for the loser of that game to get in, you want BYU to go undefeated so if we were to make the championship game at 12-0 and lose, our argument of "we only lost to a 13-0 BYU" would be strong.
Alternately, if we don't make it, we want BYU to go 12-0 so if they lose they still have a chance of getting a playoff berth as a 12-1 team.