I would argue that the monster's movement is interrupted and does not move adjacent toward the fighter. The polearm already has reach - so it *can* hit the monster in the square that it starts in. It then attempts to enter the square, and the OA fires, interrupting the action. If it is a hit, Combat Superiority kicks in and stops the movement. Since this is an interrupt, and OA's occur before the action is finished, this movement is stopped in the starting square. The monster can then use additional actions to keep moving - but this particular move has ended.
How does this play out? Well, no monster can strictly move up to a pole-arm gambit fighter and make an attack without risk. They have to risk taking the OA. If the OA misses, the fighter grants combat advantage - so there is a downfall of doing this. Tactics wise, all the mobs have to do is move in range, and then shift-attack the next turn to safely dodge the OA (since OA's specifically do not target shifts (pg290 - moving provokes).
How does this play out? Well, no monster can strictly move up to a pole-arm gambit fighter and make an attack without risk. They have to risk taking the OA. If the OA misses, the fighter grants combat advantage - so there is a downfall of doing this. Tactics wise, all the mobs have to do is move in range, and then shift-attack the next turn to safely dodge the OA (since OA's specifically do not target shifts (pg290 - moving provokes).