If a creature the defending player controls can’t block Ochran Assassin for any reason (such as being tapped), then it doesn’t block Ochran Assassin. If there’s a cost associated with having a creature block Ochran Assassin, the defending player isn’t forced to pay that cost, so it doesn’t have to block in that case either.

Remember that a source with deathtouch must deal damage to a creature for that creature to be destroyed. If five creatures block Ochran Assassin while its power is still 1, only one of them (of Ochran Assassin’s controller’s choice) will be the one dealt damage and destroyed. You’ll have to raise Ochran Assassin’s power to destroy more than one creature.