I'm curious about moving in threatened squares, say I'm a fighter and I'm already in a square fighting a monster with normal melee range and I move to the opposite side of him so that an ally can move where I was and gain combat advantage against the monster. Do I provoke an Opportunity Attack because I left a threatened square or do I not since I still within the area of his threatened reach? Or, to put it another way, is it leaving the square or leaving or leaving his reach that gives him the opportunity attack or is that my DM's choice?
Opportunity attacks are interrupts triggered by attempting to leave an adjacent square
"Leaving an adjacent square to enter another adjacent square" is just as much "leaving an adjacent square" as is "leaving an adjacent square to enter a non-adjacent square." It's not about leaving his reach entirely, it's about leaving a square he threatens.
If I'm leaving an adjacent square, that's all that's needed to trigger the OA. The attacker doesn't care where I'm going, just if the square I'm leaving is one he threatens.
[Of course, each creature only gets one opportunity action per turn, so even if I move through three of his threatened squares on my turn he can only make one opportunity attack against me.]