Its a feebie.

You can take one AoO a round unless you haver the Combat Reflexes Feat. The allowance resets itself when at the start of your turn. If you have not had a turn yet, you are flatfooted and can't make an AoO (unless you have Combat Reflexes, in which case you get your allowance before your first turn.)

AoOs always pre-empt the thing unless the text of the thing that provokes the AoO says otherwise: for example the Hold The Line feat allows an AoO against people entering a square you threaten: if it wasn't worded carefully you would not be able to reach the character when you took your attack as they wouldm, by defintion, be just out of reach. This is particularly important when trying to do a Trip attack as an AoO provoked by someone trying to stand up from prone: you can't because at the time you gotn to trip them they were still prone.

The only addition I havce is:

AoOs always occur at your best attack (if you have a BAB of 5 or more, use your full BAB). You are, however, subject to any penalties/modifiers taken in the previous round of combat. E.G. if you used Power Attack, you're still using power attack for the same value. If you used Combat Expertise, you still have that penalty. If you used two weapon fighting, you still are considered to be doing so. Etc.

1: They do pre-empt the action that causes them in most cases, in other cases like the standing up from prone they are noted.
2: No, an AoO does not always interrupt. Most combat options, like disarm/trip will have wording to explain what happens. Movement would not be interrupted unless the AoO resulted in a trip or status change that would then prevent the action. (ie. Death)
3: AoO's do not take the place of your normal attacks. Most characters get one/round unless a feat or special ability says otherwise.
4: The AoO can occur at any time during the round, but again usually only once a round and only once per infraction.

For Example: You have 4 AoO's a round due to combat reflexes and a 16 dex. A mage casts a spell next to you, you may attack him once and only once for casting a spell in a threatened area. Assuming the attack connects he will have to roll a concentration check to avoid loss of the spell.The mage then moves away from you, you may then AoO him again, once, for leaving a threatened square. Assuming this attack hits you deal your damage but then the mage can continue his movement, as long as he is still able to move. You still have 2 AoO's left until the start of your next turn at which point it will refresh to 4.

(assuming the spell and/or the movement provokes an AoO -- e.g. the spell is not a swift action or the mage does not cast defensively, the movement is not a 5 ft step, the mage does not tumble while moving, etc.)

This is one of those things I have against AoOs - there's no succinct rule as to what does and does not provoke an AoO. It leaves me feeling like I might have to run games with the book open to that page if I choose to use them.

There are 4 things that are involved when deciding if something provokes:

In order to provoke, the action must take time (free/swift actions do not provoke).

In order to provoke, the action must either require concentration -or- it must not cause an immediate threat.
(casting a spell or using a Spell-like ability requires concentration. Getting something from a bag or putting an item away isn't threatening. Drinking a potion isn't threatening. Leaving a threatened space isn't creating a threat.
On the other hand, spell-trigger items that are purely mental (wands, other items) do not require concentration, Supernatureal and Extraordinary abilities do not require concentration. Drawing a weapon creates a threat. Entering a space creates a threat.)

Non traditional attacks (trip, Sunder, Disarm, Grapple) provoke without the relevant feat.



You really should use AoOs. It is a significant balancing factor, in the long run, and avoid speople doing Stupid Things as they shouldn't.

There are only 10 of AoO items. It fits on a 3x5 card, if you need toi print them out. And considering the combat system is all part of the System Reference Document and Open Gaming Content, it is available online in several formats, already. (http://www.d20srd.org/srd/combat/actionsInCombat.htm)

ObNit: Free actions 'rarely' incur attacks of opportunity (from the same page you linked to)

There is nothing inherent to free or swift actions, as far as I know, that make them not incur AoO's. Actions that don't provoke are usually specified as such.

I agree though. The list of actions that provoke is pretty comprehensive and AoO's should be used in games. Without them some actions and some classes become a lot more powerful and less checked.

You really should use AoOs. It is a significant balancing factor, in the long run, and avoid speople doing Stupid Things as they shouldn't.

Not to quibble the point, but doesn't "stupid things" imply a generally-accepted style of play? I would think a more cinematic game would reduce penalties for doing stupid things in the name of having fun. =)

I'm still out to lunch on AoOs. I understand the balance factor, but when it gets in the way of my enjoyment (and my player's), I'm inclined to pitch it. Having to think THAT tactically about combat isn't something all my players are inclined towards... I'm not certain even I am, all the time.

Removing AoOs has a lot of side effects on the system.

If movement doesn't provoke and you don't need to concentrate to cast spells in combat, you've severely weakened the skill system. The need for casters to take ranks in Concentration is severly reduced, and Tumble becomes a unnecessary skill.

No AoOs mean that you can always move in combat. And Reach weapons are no-longer scary things; you can just move inside their area of affect without any concerns. (Now, admittedly, the new Star Wars RPGs does not have movement AoOs, but it also doesn't have many reach weapons, and characters only get 1 attack per round, regardless of level, so combat is already massively changed).

No AoOs means that archers can shoot people standing next to them without reprocussions, and charging the archer doesn't make them switch to melee weapons. It also weakens some PrC abilities. Ditto with casters, although most have high enough concentration checks as not to need to worry about casting in melee.

I can see the problems with it, but most of those issues are related to positioning. My personal preference, however, is for positioning to be a optional factor in combat - abstract combat, IOW.

D&D doesn't really have an official set of rules for running combat abstractly, as far as I know; BESM d20 simply does away with everything positioning/movement-related, and I'm not adept enough with d20 to know what other side effects this might have. I've noticed that there's an AWFUL lot of feats that seem to revolve around AoOs, and I'm trying to figure out if it's possible to reconcile AoOs into some other sort of rule that works abstractly. (Hence the timing question; since AoOs are pre-emptive, converting them to counter-attacks (for example) kind of misses the point.)

True enough, trying to keep things simple at first is all. There are always exceptions to the rule.

Now I have an image of a doddering old mage with a staff doing a tuck and roll to avoid the oncoming attack from the raging GreatAxe wielding Orc Barbarian.

Attacks of Opportunity are confusing as hell at first, but all you really need to remember is this:

In melee combat, is one person doing something that will give his opponent an advantage? If so, then that opponent probably gets an attack of opportunity.

Attacks of opportunity are just an abstract way of dealing with someone doing something that momentarily relaxes their guard. (Stepping away from combat, standing up from prone, etc.)

If you take the trouble to learn and get used to them, they really enhance the excitement of combat, in my opinion.