I heard this from a friend: put it simply, the theory goes that Otonashi was the original creator of the afterlife who went insane and turned himself into an NPC. This would make him the first to die of the SSS members, which means his heart could've been given to Kanade, despite the fact that he seemingly arrived to the afterlife after her. When he awoke in the first episode, that wasn't him entering the afterlife, but his return from being an NPC, much like how Takamatsu had returned after the shadows had been defeated.

What do you guys think?