Theory of mine from a while back: Oden's still around, just in the form of a "statue" of sorts...one infused with his dying will. If blades can become black ( or red even ), why not people as well, right? If so, I'd imagine Kaido and/or Orochi certainly kept it around as a trophy of sorts.
So Momo and Hiyori can have one last emotional chat with dad before the end by means of the Voice of Blah Blah Blah. Maybe Luffy as well, if he really is the CHOSEN ONE.