If I make an opportunity attack with some combination of riders that will cause a hit to stun (or if the victim has already taken some action, daze) can the target of my attack continue their movement?
Opportunity Attack
Moving Provokes: If an enemy leaves a square adjacent to you, you can make an opportunity attack against that enemy. However, you can’t make one if the enemy shifts or teleports or is forced to move away by a pull, a push, or a slide.
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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.
Immediate Actions
Interrupt: An immediate interrupt lets you jump in when a certain trigger condition arises, acting before the trigger resolves. If an interrupt invalidates a triggering action, that action is lost.
So, which takes precedence? The daze or stun invalidating the movement? Or the comment that after the opportunity action the creature resumes its action?
Best Answer
It doesn't say that the target gets to complete its action, only that it gets to resume it. However, now that the target is stunned (or, as you point out, dazed, if it has already taken another action), the action it was performing is now invalid, so upon resuming the action said action now simply fails. I think this is what they were trying to express with that last bit about "If the target is reduced to 0 hit points...", but they could have been more general to make it more clear.
The same would happen if an AoO were to render a target immobilized, for example -- it now can't move, so its movement action now fails.