AoO while casting 1 round spell?

[RangerWickett]

If you cast insect plague, which takes 1 round to cast, and someone tries to interrupt you by initiating a grapple, are you allowed to take an AoO, or would doing so disrupt your spellcasting?

 

[irdeggman]

While casting you no longer threaten anyone (hence why people can make an AoO on you) - so you can't take advantage of things that require you to threaten. Like an AoO on a grappler.

 

[BlackSeed_Vash]

Check out the Special part of the Concentration skill. It says,

You can use Concentration to cast a spell, use a spell-like ability, or use a skill defensively, so as to avoid attacks of opportunity altogether. This doesn’t apply to other actions that might provoke attacks of opportunity. The DC of the check is 15 (plus the spell’s level, if casting a spell or using a spell-like ability defensively). If the Concentration check succeeds, you may attempt the action normally without provoking any attacks of opportunity. A successful Concentration check still doesn’t allow you to take 10 on another check if you are in a stressful situation; you must make the check normally

In other words, it is like tumbling past an opponent.

 

[Vegepygmy]

If you cast insect plague, which takes 1 round to cast, and someone tries to interrupt you by initiating a grapple, are you allowed to take an AoO, or would doing so disrupt your spellcasting?

"While casting a spell, you don’t threaten any squares around you." PHB, page 143.

So no AoO (as irdeggman already said, but I thought you'd like the cite).

 

[javcs]

While casting you no longer threaten anyone (hence why people can make an AoO on you)

Not quite - say I'm unarmed, without any class abilities or feats that would let me threaten with my unarmed strike - I don't threaten anyone, say I'm just standing there, doing nothing, do I provoke an AoO? No, I've done nothing to provoke an AoO.

It's not the fact that you don't threaten that provokes the AoO, it's the mental concentration/focus that provokes the AoO.

Concentration doesn't let you take AoOs with while casting a spell with a casting time of one round, it merely prevents you from provoking AoOs from those threatening you.