An opportunity action is similar to an immediate interrupt, but it can be taken once per turn, rather than once per round.
Trigger: Each opportunity action—usually a power—defines its trigger. The one type of opportunity action that every creature can take is an opportunity attack.
Someone Else’s Turn: A creature cannot take an opportunity action on its own turn. The action interrupts some event on another creature’s turn.
Once per Turn: A creature can take no more than one opportunity action per turn (but it can take one on each other creature’s turn).
Interrupt: Just like an immediate interrupt, an opportunity action interrupts its trigger, taking place before the trigger finishes.
Published in Player's Handbook, page(s) 268, Rules Compendium, page(s) 196.