Infiniti, what situation in the rules could possibly cause an AoO on an action after it has begun besides spellcasting? In the example you gave of the potion, that Ogre wouldn't get an AoO on you, because you didn't take the action to drink the potion in threat range, and Readied actions allow the Ogre to hit you before you started drinking, so no AoO there either. Besides, drinking a potion doesn't take more than a standard action, so nothing else can happen during that action--beginning, middle and end are all the same in that case.