Couldn't find an answer to this one in the archives - oddly.
Richard
A disabled character with negative hit points recovers hit points naturally if he is being helped. Otherwise, each day he can attempt a DC 10 Constitution check after resting for 8 hours, to begin recovering hit points naturally. The character takes a penalty on this roll equal to his negative hit point total. Failing this check causes the character to lose 1 hit point, but this does not cause the character to become unconscious. Once a character makes this check, he continues to heal naturally and is no longer in danger of losing hit points naturally.
Nothing in either of those about not being able to take attacks of opportunity, so do it all you want.
Be aware, though, if disabled, that taking that AoO will most likely be ruled a "strenuous activity" and cause you to lose a HP and fall unconscious after its resolution.
Having exactly this question in a PBP game right now; the issue being that I'm playing an Orc (so I can move or standard while staggered _and keep going_)... I think the question boils down to something more discreet though:
"What kind of action is an attack of opportunity?"
A disabled character may take a single move action or standard action each round (but not both, nor can he take full-round actions, but he can still take swift, immediate, and free actions).
(and about the same for staggered).
Attacks of opportunity are defined in the PRD:
Attacks of Opportunity
Sometimes a combatant in a melee lets her guard down or takes a reckless action. In this case, combatants near her can take advantage of her lapse in defense to attack her for free. These free attacks are called attacks of opportunity. See the Attacks of Opportunity diagram for an example of how they work.
Threatened Squares: **OMITTED**
Provoking an Attack of Opportunity: **OMITTED**
Making an Attack of Opportunity: An attack of opportunity is a single melee attack, and most characters can only make one per round. You don't have to make an attack of opportunity if you don't want to. You make your attack of opportunity at your normal attack bonus, even if you've already attacked in the round.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).
So: we have AoO's described above with some choice words bolded and italic. It says it's a "free attack," a "single melee attack," that it "interrupts" and happens "immediately." (It does not explicitly say "An attack of opportunity is a _________ action" anywhere that I can find).
Melee attacks are standard actions; if this is a standard action ("single melee attack") then a staggered/disabled character cannot take an AoO if they already moved/did a standard action this turn. However, players cannot take standard actions on someone else's turn, so I don't think that AoO's are standard actions. (In the case of disabled characters they are clearly strenuous and would make you bleed).
AoO's are called "free attack[s]," so perhaps they are free actions. However, as with standard actions, free actions (other than talking) can only be taken as part of another action (which means only on your turn or when you get to do an immediate action on someone else's turn).
Lastly we have "interrupt" and "immediately," which opens the possibility that AoO's are immediate actions. This also doesn't quite pan out, however, because using an immediate action not on your turn means that you "cannot use another immediate action or a swift action until after your next turn if you have used an immediate action when it is not currently your turn (effectively, using an immediate action before your turn is equivalent to using your swift action for the coming turn)." Clearly taking a regular AoO does not prevent anyone from using their swift action on the following turn, not to mention that Combat Reflexes lets a player take multiple AoO's in a single round...
So I think the answer to this question (unless we get an official response) will remain in the realm of GM fiat/RAI.
So: if you're the GM, how do YOU deal with this?
A quick aside: Disabled and Staggered are similar in the actions you can take, but disabled = at 0 HP or negative and conscious. In the PBP game that spawned this question for me, I'm playing an Orc, and so Ferocity lets me keep acting even at negative HP--so if I take standard actions/strenuous actions, I bleed... but I can keep on acting (one move or standard per round) until -CON.
I took the AoO that the caster provoked, and asked the GM (in the discussion thread) to rule on 1) whether I have used up my "move or standard but not both" for the next round, and whether it made me bleed an additional damage as a strenuous act (I failed to stabilize on my last turn so I bled for 1 already this round).
My answers to the above situation (were I the GM) would be: 1) You can take the AoO, it doesn't use up your standard from next round, but it's strenuous so bleed another HP Mr. Ferocious Orc. =p
"What kind of action is an attack of opportunity?"
It's none of the listed action types. Normally, this would mean it's not an action, but that would also mean you could AoO while dazed, so it was clarified that making an AoO is an action, in that it's an act, something you do, though it's not a listed Action from the table.
Since all other attacks are a action, are attacks of opportunity a action?First of all, the way this question is phrased, there's NO WAY I can answer it without a legion of rulesmongers picking it apart like a swarm of starving jackals picking apart the seven-day dead sun-baked carcass of an elephant.
"Action" is not a defined term by the rules—it covers ALL of the possible things you can do in the game. Some of them are full-round actions, some swift actions, etc. But the word "action" itself doesn't mean anything.
So yes, attacks of opportunity are an action, but so is picking up a dropped sword, rubbing your eye, building a castle, tripping over a root, blinking, remembering you just blinked, stabbing a rulesmonger, and exhaling. Along with every other thing a character does, consciously or unconsciously, in the game.