Also, I don't think that Itachi's sword killed him. We see a snake get burned by Amaterasu during the end of the Sasuke and Itachi fight. This is what I presume to be Orochimaru's final struggle to survive and that could make his resurrection possible because then he really isn't stuck in some genjutsu world or whatever. That is assuming that Kishi wasn't simply wasting resources to randomly draw snakes right after we saw the snake and resurrection themed villain get killed.