Whenever Ogre Marauder attacks, it gains "Ogre Marauder can't be blocked" until end of turn unless defending player sacrifices a creature.
2/1/2005 Ogre Marauder’s ability requires the defending player to sacrifice a creature as the ability resolves, or Ogre Marauder can’t be blocked for the rest of the turn.
2/1/2005 Attacking with Ogre Marauder a second time would cause a second trigger. If no creature was previously sacrificed, the Marauder already can’t be blocked. Sacrificing a creature for the second trigger will not nullify the first.