Does hitting an opponent with an opportunity attack stop their movement?

Question

Disrupting Actions

Various abilities and conditions, such as an Attack of Opportunity, can disrupt an action. When an action is disrupted, you still use the actions or reactions you committed and you still expend any costs, but the action’s effects don’t occur.

I'm confused about the part where it says "the action's effects don't occur".

Attack of Opportunity

Trigger: A creature within your reach uses a manipulate action or a move action, makes a ranged attack, or leaves a square during a move action it’s using.

For example, a creature uses the stride action to move 20ft back. The monk, who was on melee range, gets an attack as soon as the move action is declared (before leaving the square). The monk hits, so "the effects don't occur". My interpretation is that the effect of that action was "moving 20ft back". However, at the same time, it says that "you still use the actions you committed".

Can someone clarify these rules? Thank you.

Answer 1

Opportunity attacks do not stop movement

The Attack of Opportunity rule continues with the following description after the trigger section, for the actual attack:

You lash out at a foe that leaves an opening. Make a melee Strike against the triggering creature. If your attack is a critical hit and the trigger was a manipulate action, you disrupt that action. This Strike doesn’t count toward your multiple attack penalty, and your multiple attack penalty doesn’t apply to this Strike.

You are not automatically disrupting the triggering action with an attack of opportunity, only when it is a critical hit and the action was a manipulate action. But it still is true that an Attack of Opportunity can disrupt an action, in that specific case (as mentioned in the Disrupting Actions section).

A move action is not a manipulate action, so you never can disrupt it with an attack of opportunity, and the Disrupting Action rule never gets invoked, not even if you happen to hit them with a critical hit. You never get to the "the effects don't occur" part, as you are not disrupting the move action in the first place. They can move away unhindered, unless you happen to kill them with your attack.

Answer 2

In general, no, a standard Attack of Opportunity (AoO) will not disrupt movement by default.

AoO's do trigger from an enemy taking a move action, or on leaving a square during a move action. So, from your mechanical example, the creature would declare he is taking a Stride action, and the monk could take an Attack of Opportunity immediately, or could wait until the creature left a square which he is able to reach. With a normal AoO, the creature's action would not be disrupted, and he would then continue his movement action as he saw fit. Or, the monk might kill the creature, which would stop it moving further. As another example, if the creature instead announced it was drawing a weapon, this is a Move Action with the Manipulate trait which triggers AoOs and can be disrupted by default. If the monk Critical Hit during an AoO from this trigger, the weapon would not be drawn. If the creature was still alive or had actions left, it could try again.

There are special abilities that change this default behaviour. One example is the feat Impassable Wall Stance where any Critical Hits from Attacks of Opportunity will disrupt triggering movement. Another is Disrupt Prey, which doesn't actually trigger an AoO, but does trigger a Strike that can disrupt movement on a Critical Hit. Stand Still is a Monk feat that also triggers an attack on a move action but isn't an AoO. I couldn't find any more, but there might be others (now or added in the future).

All of these examples say they 'disrupt the triggering action' on a critical hit. They also trigger on leaving a square during a move action, or on the move action itself. With any of the example abilities here, an attacker potentially COULD disrupt the movement action. In these cases, the creature has the action end at the point of triggering, and undoes the triggering movement out of that square. If the creature still has any actions remaining, it could attempt to move again. Given that it is almost impossible to have more than one reaction, the monk would be very unlikely to be able to react again, although other allies might be able to.

So, if the Monk in your example has access to any of these abilities, if the creature declared a Stride, the Monk could attack either when the move was announced or when the creature left a square he could reach. With a Critical Hit, the rest of the move action would be cancelled. Any movement that had already happened before the AoO was triggered would still stand. In this way, the Monk could wait until the creature was in a better position, such as a flank, before striking.

The last little wrinkle to note is that the Step move action specifically doesn't allow any reactions to trigger, but it does use an entire move action to move only 5 feet.