Has the conditions of Combat Superiority been satisfied?
If the fighter hits with the OA, what does he get to do?
Doesn't push require the monster to be moved away from the fighter? That might be difficult.
He could push himself out of the gullet of the creature
Having said this, I think what I may like less about the swallowed defender using his powers is the fact if my dwarf fighter gets swallowed he can still swing his waraxe inside the beast. If they had said only attacks with weapons that are one-handed and have the off-hand property I'd be a bit more happy. In fact, I think I'll be house-ruling that one.