Feats on attacks of opportunity.

[sword n' board]

can you use feats on attacks of opportunity? (i.e. vital strike or power attack or two weapon fighting) if not, please explain why.

[blackbloodtroll]

Power Attack, yes. You choose to use it before any attack, and it lasts your whole turn.

Vital Strike, no, it is it's own standard action.

Two Weapon Fighting is something you can only do as part of a full attack, whether you have the two weapon fighting feat, or not.

[Elbedor]

Off the top of my head, it would seem that any Feat which can be performed as a melee attack (as opposed to a Standard Action) could qualify as an AoO.

[TGMaxMaxer]

It depends on the Feat. If it takes an action (like vital strike, cleave, things like that), then no.

If it just modifies an attack (power attack, imp/gr trip, sunder, disarm, etc) then yes.

[Raymond Lambert]

Remember that you choose to use power attack and/or combat expertise on your turn. If you did on your turn, those modifiers are locked in till your next turn, you cannot turn them off till your next turn. If you did not on your turn, you cannot simply turn it on when a AoO becomes available.

[Gauss]

Raymond Lambert, for Power Attack, what do you base that on?

Power Attack has no language that states it must be used on your turn only, only that you must choose to use it before making an attack and that it lasts until your next turn. An Attack of Opportunity qualifies as an attack and does not violate it lasting until your next turn.

Note: You are correct regarding Combat Expertise because it has language that it must be used as part of an attack or full-attack action and thus can only be declared during your turn.