Can you make an attack of opportunity before you have acted in that round combat?

Question

We had a recent question in my party today regarding when you can make an attack of opportunity.

One group believes that you only threaten squares after you have come up in the initiative order for that round. Therefore you don’t threaten squares and can’t make attacks of opportunity until after your turn.

The other group believes you can make an attack of opportunity when anyone leaves your threatened square (one per round unless otherwise stated) before or after your turn.

Can you make an attack of opportunity at any time or only after your turn?

Answer 1

You make an attack of opportunity as soon as it is provoked, regardless of whose turn it is; but you cannot make one if you are flat-footed, which includes if you haven't acted yet in this combat encounter.

According to the rules on Attacks of Opportunity:

An attack of opportunity "interrupts" the normal flow of actions in the round. If an attack of opportunity is provoked, immediately resolve the attack of opportunity, then continue with the next character’s turn (or complete the current turn, if the attack of opportunity was provoked in the midst of a character’s turn).

And according to the flat-footed condition:

A character who has not yet acted during a combat is flat-footed, not yet reacting normally to the situation. A flat-footed character loses his Dexterity bonus to AC (if any) and cannot make attacks of opportunity.

If I understand correctly, the first group appears to be misreading that flat-footedness resets from round-to-round; it doesn't. When combat begins, you're flat-footed. Once your first turn comes up, you are no longer flat-footed for the rest of the combat, and can make attacks of opportunity as they trigger.

Answer 2

Once you've taken your first action within combat, you always threaten squares whether before or after your turn, for every following round.

Player's Hand Book
Page 135

During combat, you threaten all squares adjacent to yours, even when it’s not your turn. An opponent that takes certain actions while in a threatened square provokes an attack of opportunity from you. An attack of opportunity is a free melee attack that does not use up any of your actions. You can make one attack of opportunity per round.

During the round you get one attack of opportunity(unless you have a feat that gives you more) to use any time one is provoked, but once you use it you don't get another until the next round when the turn order that was determined by initiative starts again.