Talk:Oscar, Knight of Astora/@comment-174.70.161.85-20140122205818/@comment-147.194.135.69-20140729233806

Anon3 has a point, DS has always been pretty vague about when an Undead stays permanently dead. At more than one point is it stated that the Undead never really die, they just eventually go Hollow and become a danger to others, but we clearly see some Undead NPCs die and stay dead. I think permanent death is only possible for an Undead once they go full-on Hollow, otherwise they'll just keep coming back.

That would explain Oscar's situation. When he gets smashed by the Asylum Demon, he probably loses hope, especially once he realizes that he's set you free and passed his mission on to you. He might realize that he's about to go Hollow, which is why he does come back, one more time, only as an enemy. Once you kill his Hollow form, he doesn't come back.