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You cannot make a 5' foot step during an Attack of Opportunity. You would have to have an item or feat or some special ability to do something like this.
Attacks of Opportunity are actually resolved the exact moment that they are provoked and even interrupt the turn of the person who provoked the AO. It takes no action whatsoever, and gets resolved before anyone can take any other actions, even immediate actions.
The only thing I can think of that you might be talking about is the feat Pin Down, which allows you to provoke AoO's on people who take 5ft steps to get away from you, OR, the Step Up & Strike feat chain, which when someone takes a 5ft step to get away from you, you're allowed to follow the target using your own 5 or 10ft step as an immediate action and make an attack that counts as one of your Attacks of Opportunity for that round.
This is what Ferka and MrCharisma are talking about, maybe this is what you're thinking?
Readying an Action
You can ready a standard action, a move action, a swift action, or a free action. To do so, specify the action you will take and the conditions under which you will take it. Then, anytime before your next action, you may take the readied action in response to that condition. The action occurs just before the action that triggers it. If the triggered action is part of another character’s activities, you interrupt the other character. Assuming he is still capable of doing so, he continues his actions once you complete your readied action. Your initiative result changes. For the rest of the encounter, your initiative result is the count on which you took the readied action, and you act immediately ahead of the character whose action triggered your readied action.
You can take a 5-foot step as part of your readied action, but only if you don’t otherwise move any distance during the round.