When a monster doesn't use an AoO, does it mean it can't take one later on?

Question

I'm surrounded by melee monsters and I want to cast a spell, but I'm afraid that I would fail the concentration either when casting defensively or when taking damage during spellcasting.

Casting the spell is more important for me than not taking damage, and I would prefer to stay in my current position if possible.

I'm thinking of this tactic: move one square (using the Move action), then

• If all the monsters attack me, that's great, their AoO is spent, I can return to my original position and cast my spell safely without making a concentration check.
• If they don't, then that means that they chose to not take the opportunity and I can use the rest of my Move action to safely move through any squares that they control without triggering another AoO from them, perhaps moving to a square that they don't threaten and cast my spell from there.

Does this work?

Answer 1

A creature can definitely choose not to make an attack of opportunity at the first opportunity, in order to attack on the next opportunity. For example, if you moved, provoking, and a creature held back, and then you provoke them again by casting a spell from a square they threaten, they could take that attack of opportunity.

However, when you move, all of that movement is one opportunity. The rules are very clear that continuing to move after attacks of opportunity have been taken does not provoke more—not even from creatures with Combat Reflexes.

[Combat Reflexes] does not let you make more than one attack for a given opportunity, [...] Moving out of more than one square threatened by the same opponent in the same round doesn’t count as more than one opportunity for that opponent.

^{(Combat Reflexes and Additional Attacks of Opportunity)}

It is not clear whether or not a creature could hold back after leaving one threatened square, and then later choose to take the attack of opportunity when you leave another threatened square during that same movement. The rules don’t really address “ongoing” provocations like that, with the exception of this one section on Combat Reflexes that says you can’t take more attacks of opportunity from one movement, and a rule specifying that you can only take an attack of opportunity when a caster begins to cast a long-casting-time spell. Anyway, there is a fairly strong circumstantial case against being able to, but I certainly allow creatures to do so in my games; I find it makes more sense and doesn’t cause problems (it’s pretty rare that a creature wants to in the first place).

Regardless, if you are able to move outside of all creatures’ threatening area, you should be safe—after all, whether they take their attacks of opportunity for your movement or not, you are no longer threatened when you cast your spell, so you have no risk of having it disrupted. If you cannot do that, and end up in a square that is threatened, moving hopefully consumed their attack of opportunity one way or the other, so you can avoid attacks of opportunity during your spell casting.

Finally, remember that casting defensively is a thing—which you may want to use regardless, as creatures may have Combat Reflexes (which would not allow them to take multiple attacks of opportunity for your movement, but would allow them to do so for both your movement and your spell casting), and even if you avoid any apparently-threatened squares, it’s possible for someone to have surprising reach, or for someone to be hidden/invisible. The Concentration check to do so isn’t trivial though, so depending on your level, the level of spell you’re casting, and so on, it may be a greater risk than the possibility of an attack of opportunity (particularly if the only way you could risk one is if a creature unexpectedly has Combat Reflexes, greater reach, or invisibility). On the other hand, if you know you are threatened, your odds of success are likely higher casting defensively than they are risking an attack of opportunity—to say nothing of multiple attacks of opportunity, at which point you almost-certainly want to cast defensively.

Answer 2

Yes and no.

You do not get more than one opportunity attack per triggering action.

If you're surrounded on, lets say 6 out of 8 adjacent squares, and you take a move action (not a 5-foot step) then any enemies that threaten you can choose to attack when you provoke by moving. If they do not make that attack when you move your first step, then they cannot make an attack later during that same movement. Once they forgo the opportunity, they lose it entirely until you do something else that provokes, like casting a spell. So you could take the first step of your movement, see that they don't attack you, and walk rings around them if you so chose (and had the remaining movement to do so).

When you start casting, and you don't cast defensively, you will provoke an opportunity attack again, and any enemies with opportunity attacks remaining (whether it is because they didn't take on earlier or because they have combat reflexes) can once again choose whether or not they wish to utilize this opportunity.

So if you were to take a move action to start walking away, and all of the enemies took an opportunity attack against you, first, you would end up taking a lot of damage, then (if you were still conscious) you could walk back up to them and cast. If any of them has Combat Reflexes, they will get another opportunity attack on you for casting.

For some simple math, here's what your odds look like for getting the spell off.

In all honesty, you're FAR better off casting defensively than trying to cast and save vs the damage.

Casting defensively is 15 + 2x spell level (for a maximum of DC 33 for a level 9 spell), while casting from damage is 10 + damage + spell level. Unless you're fighting kittens, any ONE attack is more likely to exceed the level of the spell you're trying to cast. For reference, here's some math on a lvl 1 spell. Either DC 17 (15+(2x1)) or (10+1+damage) and the likelyhood that an attack is going to be less than 6 damage is rather low, even at low levels.

At level 1, you should have a concentration bonus of somewhere around +6 (+5 stat, +1CL) giving you exactly 50% odds to cast defensively successfully, meanwhile getting hit by ONE dude cause will average 4 damage at level one, giving you about a 60% chance at success. And that's assuming that only 1 out of those 6 guys hit you. If all 6 hit a) you're dead, and b) depending on your DM you may have to either roll 6 times (once per hit) OR, he could total all the damage first, the make you roll vs THAT DC, so if they averaged 4 damage on a hit, that would be either 6 DC 15 checks, which are bad odds, or 1 DC 35 check, which you'll fail automatically (no auto successes on 20 on skill checks, and you can't roll a 29 on a d20).

Once we hit level 8, our defensive DC for a level 4 spell has only gone to DC 23, meanwhile your concentration should be now somewhere around +15 (+7 base stat, +8 CL) giving you a 60% chance of success, assuming you haven't gotten anything to further boost concentration checks, like traits, feats, class features, items, or racial bonuses. Whereas if you want to tank some hits, you're now looking at an average of 13 damage on a hit, giving you a starting DC of 27, which is already much harder than casting defensively. And if you take more than one hit, you're basically guaranteed to fail.