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.
Your nearby enemies learn the hard way that it is unwise to ignore you.
Opportunity Action Personal
Trigger: An enemy subject to your defender aura either shifts or makes an attack that targets an ally of yours, without targeting you or an ally who has an active defender aura.
Effect: You make a melee basic attack against the triggering enemy and deal 1d8 extra damage to it.
This is not my first DM homeruling against PC character making an opportunity attack triggered by an enemy's opportunity attack against an ally that provokes an opportunity attack from an enemy subject to my defender aura. I know that I'm correct in my assumption and understanding of the rules as written (RAW). However, I haven't found any discussion in these forums that validates my argument. The trigger for Vengeful Guardian states "when an enemy makes an attack..." An opportunity attack is an attack, right?
Thank you for any feedback.