I have an Ally in play and i am engaged with an enemy. For my action i play an new Ally (the first one will be discarded). So when the monster makes his attack of opportunity i choose the Ally to take the damage. But which Ally? The first or the new one??
The attack of opportunity takes effect before the first Ally is replaced or after?
You need to forget about the second ally for the duration of resolving the attack of opportunity. That only gets placed into your 'in play' area once the AoO is resolved.