Once per turn effects can become eligible for use again on each combat participant's turn (friend or foe), so the main question here (judging by the comments) is if you can use it on an OA and a Readied Action on the same triggering creature's turn. The answer is:
No, Readied Actions do not grant an additional turn for sake of Once Per Turn effects
Opportunity Attacks act as an interrupt to the action they provoke from, thus causing them to exist on the turn of the creature who triggered them.
As for Readied Actions, see this quote:
Immediate Reaction: Using a readied action is an immediate reaction, so it takes place right after the trigger finishes.
So a Readied Action is an Immediate Reaction, now over to this from the Immediate Action rules:
Someone Else’s Turn: A creature cannot take an immediate action on its own turn. The action interrupts some event on another creature’s turn or responds to that event.
This means that a Readied Action happens on the turn of the creature who triggered it. You can use your once per turn effects; however you may still only perform one on that creature's turn. If your Opportunity Attack triggers first and you use a once per turn effect there, it's already been used for that turn and you will have to wait for that creature to end it's turn for you to be able to use it again.
Note that if your Opportunity Attack and Readied Action are triggered by two different creatures, then you're able to use the same Once Per Turn effect on both.