Let's say "John" is a fighter who has selected a number of feats to focus on tripping his enemies, including Greater Trip, and Combat Reflexes, and wielding a Reach Weapon while under the effect of a potion of Enlarge Person.
Greg is approaching John, and moving from 20ft to 15ft, is tripped by John's AoO, then hammered by Greater Trip.
Greg takes a move action to stand, granting John a second AoO.
Can John use this AoO to trip Greg again, thus keeping him prone?
(Using Greater Trip to provoke an AoO to prevent Heat Death of the Universe. Boredom may still be an unfortunate side effect)
Best Answer
No. Attacks of opportunity are resolved prior to resolving the triggering event. Standing up provokes an AoO, and sure, you could use it to trip him again, but he's already prone (technically, he's in the process of standing up, but since he hasn't finished the transition yet, you still get the bonuses for him being prone). After your AoO resolves, the victim then finishes standing up. (However, the victim is probably fairly out of luck in this particular scenario, since he's going to get tripped again if he moves towards or charges the trip-fighter, and advancing via 5-foot steps isn't going to work unless the trip-fighter can't take them away from the victim for whatever reason.)
PFSRD, FAQ:
Furthermore, the parts on attacks of opportunity and concentration heavily imply that an attack of opportunity happens during the triggering event, but without preventing the triggering event unless otherwise stated (like in the case of a spell: just being hit - or even tripped - during spellcasting doesn't necessarily end the spell, if the caster can maintain concentration). To me, this means tripping someone while they're starting to get up just slows them down, not enough to warrant it changing how their turn plays out. (A readied action could be set to trigger after someone finishes standing up, though.)