On one hand Oden has to be dead, he is in an important character and his death is important for the plot and the inherited will concept (open Wano, Momo fight for Wano, if now he was alive this all will take a huge blow). Also the last we see of him was him dying with a smile on his face (like a good D would do), so underling the fact that he is dead (yet Oda didn't show his dead body so you can never know lol, he could just be back as Poden like it was already adressed).
On the other he has got many weird resemblances with Oden (ribbon on the back, shackles on his wrists, height seems the same, mentioned he has been waiting for him the whole time, why has he a mask?).
So let'see. The cheapest theory I can come up with is that Toki send him to the future when he was submerged and nobody could see his body and sent him to a very recent moment in this time (some days ago at best) then he took Kaido's son identity (but the theory is already full of holes). Or maybe he was still alive after emptying the oil pod, Kaido alone discovered it, praised his strength and kept him alive (amnesia mode) as his son not telling Orochi who really the guy is (and that is why he wears the mask)(yet this theory too has plenty of holes like why is he still keeping the shackles?)