If a character moves away from a square adjacent to a monster, the monster gets an OA. Good. What happens after the attack is resolved? Can the character continue moving?
So far, I've houseruled that if the attack misses, the character can continue his movement, but not if he is hit. I'd like to know if there's something official.
From Opportunity Action,
Interrupts Target’s Action: An opportunity action takes place before the target finishes its action. After the opportunity attack, the creature resumes its action. If the target is reduced to 0 hit points or fewer by the opportunity attack, it can’t finish its action because it’s dead or dying.
A normal OA will not interrupt movement. Even if the creature is hit, it can continue it's movement as normal.
That said, it is possible to stop the movement, but you'd need a power or ability that explicitly says so. For example, the stance Defend the Line would allow the fighter to slow the enemy on an OA. The creature would then only be able to move a total of 2 squares for his current move action. If he has already moved 2 or more squares, he stops in the square he provoked the OA from.
There's also the fighter's Combat Superiority which stops movement altogether.
Combat Superiority
You gain a bonus to opportunity attacks equal to your Wisdom modifier. An enemy struck by your opportunity attack stops moving, if a move provoked the attack. If it still has actions remaining, it can use them to resume moving.
PHB1 on p290 goes into detail on opportunity attacks. As far as I read it, unless an attack that is being used for the opportunity attack specifically mentions that it stops the character, then they can continue moving.
For example, the fighter's Combat Superiority ability reads
An enemy struck by your opportunity attack stops moving, if a move provoked the attack. If it still has actions remaining, it can use them to resume moving.
The Compendium lists:
Interrupts Target’s Action: An opportunity action takes place before the target finishes its action. After the opportunity attack, the creature resumes its action. If the target is reduced to 0 hit points or fewer by the opportunity attack, it can’t finish its action because it’s dead or dying.
Conceptually, the attack happens while they're moving - unless the attack does something to prevent the movement (dropping the target is listed, I'd also say adding the slowed or immobilized conditions would work as well), the target finishes the action after the response.
As a DM I have sort of a house rule that seems to make sense.
If the creature or PC is attacking and provokes an opportunity attack we roll the opportunity attack and if the roll is a Critical Hit we stop the movement and allow them to use the rest of their actions or even an Action Point to finish out their turn, if they have an action point.
This has worked for my group for awhile now and I have seen other groups use this method in Encounters as well.
