I think the rules are pretty clear cut. There is one scenario I thought about where it can cause confusion though: Say Aaron Rodgers throws an INT to Brian Dawkins in the first possession of OT. Dawkins tries to return it but then fumbles and the Packers recover. The Packers eventually kick a FG. Is the game over? I'm pretty sure, yes. It may not happen at all, but an onside kick to begin OT may cause confusion as well. I heard if a team opens OT by recovering its own onside kick, a FG can win the game because the other team had an opportunity to take possession. It might be beneficial to try an onside kick. If they kick a field goal you still get the ball but if you recover it than you can go win the game than. You're also making it a whole lot easier for a TD to be scored against you.