If I hit an opponent with an AoO and Large and in Charge, it is pushed back to the square where it started from. If I also have Knockback, do I calculate the bull rush result from its original square?
Knockback does not say anything about the actual squares in question, while Large and in Charge does. That means that Large and in Charge should resolve first (while the creatures are in the positions it requires), and then yes, Knockback should be resolved from there. Large and in Charge effectively adds 5 feet to the distance you are knocking the target back.