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.
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.