some points as to why i dont think its oden (if it wasnt obvious):
1. Oden in the chapter came from behind a door from another room, how would someone like oden be able to walk around kaidos hiding spot.
2. lets say toki sent him 20 years into the future, then his wounds should not be healed right now. idc if youre a one piece character, oden should be literally charred right now.
3. Oden came in with some smoke trails, when onimaru transforms he also brings smoke trails.
4. lets say my previous points were all wrong and its actually oden, whats even the point? zoro having enma, yamatos existence, momo becoming shogun. it would all become pointless. if oden is as how he is then he should be able to fight kaido right now and make things much easier.
but lets say its oden, then oda must have a damn good explanation for this.