If Florida State is going to play for the national title this year, it must beat Oklahoma.
It's not that a one-loss team can't or won't finish at the top of the BCS standings, but Florida State's schedule does not lend itself to playing enough ranked teams that it would be able to bounce back from a loss to the Sooners in the BCS standings. Brad Edwards addresses this topic in his BCS Minute, and I agree. If FSU loses to Oklahoma, it's possible the only other ranked opponent it would face would be in the ACC championship game.
A loss to Oklahoma wouldn't be the end of the season for the Seminoles, but it certainly would all but end their national title hopes for 2011.