In Dungeons & Dragons 3.5 there is such a thing as Attack of Opportunity, or AoO for short. A nifty little trick in the game: anytime anyone does an action which takes their concentration away from you, you can bash them on the head. Only once per round though…unless you have Combat Reflexes, then you can AoO as many extra times a round as you have a Dexterity Bonus. Wizards of the Coast have two handy articles about the AoO rules and what they can do, though it doesn’t really expand on their full potential. The implications and possibilities of AoO didn’t really hit me until I read a certain Order of the Stick comic.
From here, ideas have began to form. Reach is a nifty weapon function for AoO. Some weapons are long enough to attack creatures farther away from you, allowing you to hit everything that rushes at you (at least those of Medium and Large [long] size, because those could touch you with their own reach before such an AoO). However, you can’t use certain weapons in close combat.
Spiked Chain takes care of this, allowing you to hit things next to you due to its manageable length. Coupled with a Large character, you have a 20 ft Reach, making the battlefield around you into your playground. Sure, it requires you to take Exotic Weapon Proficiency for the Spike Chain, but if you’re working on an AoO build, it’s worth it.
If you manage to down an opponent with enough damage from an AoO and have the Cleave, you can hit someone else at the same bonus. Doing this requires a high damage output though, considering downing an opponent and having another in range to hit is very rare in higher levels of play. However it should go, Cleave can only be used once per round…unless you have Great Cleave, then there is no limit to the number of times you can Cleave through enemies.
Above 10th Level, the Rouge has the chance to learn Opportunist, which allowed the Rogue to make an AoO against an opponent who has just been struck for damage in melee by another character. Once per round, of course, though still part of the of the pool which can be pulled from Combat Reflexes. Just another means to get more hits in, of course, and it’s a large investment as well, considering that’s 10 levels or Rouge you need to take in order to get it. Still, the Sneak Attack +5d6 is nothing to laugh at.
Going beyond core rules, there is a Feat in Fiendish Codex 2 called Supernatural Instincts which allows an AoO whenever a threatened creature uses a supernatural ability. Considering there are a countless number of monsters who have supernatural abilities, especially in higher levels, this is intense.
Now throw in the concept of tripping with all this. If using the Spiked Chain mentioned, you can easily trip in conjunction with Improved Trip, and after knocking an opponent to the ground you can immediately attack them as if you hadn’t used your attack for the trip. Just that is awesome, especially when the opponent stands back up, invoking an AoO, and with that Combat Reflexes going round, and maybe a Cleave if they drop, throwing in a Power Attack to make sure they do, and bonus because the Spike Chain is a two handed weapon…fun times.
You don’t need to just attack in an AoO. You could disarm (take away an opponents weapon), grapple (hold an opponent down), sunder (destroy an opponents weapon), or even trip. If you are holding the charge of a spell or have a touch spell still active, you can make a touch attack with said spell.
And if you should ever go Epic (above Level 20), there are some handy Feats one could take as well, which make the above options even more powerful. Improved Combat Reflexes gives no limit to the number of AoO you can have. Sneak Attack Of Opportunity makes any AoO a sneak attack (suddenly Rouge’s Sneak attack is awesome). Spell Opportunity allows you to AoO with spells (should you be inclined to such a route). Spellcasting Harrier allows you to attack casters even if they cast defensively.
Of course, one must always follow the golden rule with AoO: you can only use each opportunity that presents itself once on any given opponent. As an example, even if you should have a reach of 20, an opponent can only be hit once a round even if it dances around you. Not really a bad thing, since it’s just a basic understanding of the rule, and something to be accepted. And where there is a rule, there exists the capability to maximize upon the potential of what it allows you. You can AoO someone who picks something up, casts a spell, and is pushed through your threatened squares. Be creative!
This is everything my months of thinking and obsessing of AoO have collected. Funny thing is, I’m sure there are still plenty of ideas and concepts I haven’t considered yet, or don’t even know about. When you consider the fact I’ve only focused on the offensive aspect of AoO and not defensive, it’s pretty much a guarantee. Makes me happy knowing as such. Or slightly mad. I take what I can get. In either case, if you’re looking for some ideas for optimized character builds based around AoO, check out this wiki, along with other ideas in general. It’s a D&D player’s dream.