Not technically true, although it 'works' under most circumstances.
You can take one and only one OA on each of your opponents turns (regardless of who is triggering the OA).
In other words, if the ranger and wizard both took a point blank ranged attack during a warlords turn (for example due to power that granted allies a Ranged Basic Attack) - you only get one OA (on whichever one you choose to attack). Not two OAs (which the above would imply).
Likewise, if the wizard attacked you on his turn and you took an OA against him during that turn and then later on, during another character's turn, you were somehow granted the ability - for example because he was granted a free attack by someone else's power - to take an OA against that same wizard you would be able to do so. The fact that you have already attacked the wizard during that round is not relevant as long as it was during some other players turn.
The restriction is on when you can take the OAs, not on who may be the target (assuming the target triggered an OA, of course).
Carl
