PF current rules - Not for the Playtest or 2.0 rules.
Can a dwarf barbarian, while raging, use a dorn-derger (from Dwarves of Golarion, as the reach weapon, not adjacent) to initiate a Bull Rush (as from the Knockback rage power) as an Attack of Opportunity when a creature exits the 10' threatened square to move adjacent to the dwarf? The Knockback allows once per round, a Bull Rush attempt against target in place of a melee attack.
The Core rules state that an AoO is a single melee attack but also states that an AoO is a free attack that interrupts the normal flow of actions. It also states that a Bull Rush is a standard action or as part of a charge, in place of a melee attack.
So the real questions are
1.) Can the barbarian use the weapon to do a Bull Rush attempt as an Attack of Opportunity, while raging, with the Knockback rage power?
2.) If you can initiate the BR attempt with a weapon as an AoO, does that take a standard action and then you forgo any additional attacks that round?
It would seem to me that the standard action required to initiate the Bull Rush would prevent it from being able to be used as an AoO.
I am in need of clarification, as a PC is attempting to use this rage power / weapon combo to the greatest effect - and also to create a perimeter around him that prevents creatures from entering adjacent squares.