Who knows.... maybe Oro was watching Obito to possibly take his body and when he 'died' Oro might have done something
You think Orochimaru is involved somehow?
According to this translation (Naruto 557 page 6)
Madara said that he would have taught Hashirama how to rise from the dead...
.... maybe Madara did a technique that allows him to rise from the dead and had to use Nagato as a vessel for it to happen.
either way I just wanted to point out...