Absent a readied action, the attack of opportunity only grants an extra attack.<br/><br/>And I am not prepared to make a blanket ruling from a particular instance.<br/><br/>Edit:  After some deliberation, I will say that, barring any statement that "the creature imposes an obstacle," moving through an enemy-occupied space should result in its success, with the caveat that the creature will be entitled to an attack of opportunity.  If the creature has combat reflexes, the creature could be entitled to several attacks of opportunity.<br/><br/>We will treat such spaces as threatened squares rather than occupied squares, with an exception being that you cannot end your turn occupying the same square as the enemy.