Oden's partially blocked (at least that's how I see it when I look at the panel) Roger's attack and took from in the front, while Kaido smacked Oden from the back of his head.
I don't think Oden is actually *fragile* since we know cheapshots can very effective but Oda not having Kaido being in hybrid or at least use aCOC despite that power being introduced in this very flashback is such a weird choice and easy slander material.