Regigigas being introduced in a different generation than Regi trio.
I'm not a big supporter of Arceus being OD, but I think it's not impossible. Anyway, since Arceus is stated as the God Pokémon and all, can be related with anything. If OD isn't Arceus, it could be related somehow anyway, or even be sort of a counterpart (specially if OD's base stat is 720 as theorized). Its outer space origin makes this connection more likely. Also, I think there could be a reason why Ghetsis was in possession of Adamant, Lustrous and Griseus orbs.
About OD, I think it could be sort of Dragon-Multitype, taking either Fire, Electric or Ice as secondary type (or none, in its "natural" form).