So easy for Kisame. Great Water Shark Bullet > everything except non chakra based jutsu, which Oro don't have. Samehada heals all injuries and Oro dies of chakra exhaustion from one shark bullet even in his Hydra form (shark bullet will just absorb his chakra and get even bigger, it's bigger than Hydra to begin with).
Oro only has a chance with Edo Tensei otherwise any other version. Of Oro gets stomped.
Why people acting like Orochimaru got a boost in power with the Zetsu body? The Zetsu body gave him no boost.
Things get tricky since the location is a giant body of water if I remember correctly.
In a neutral location Orochimaru would win, since he can easily escape Water Dome and render Kisame's strongest Water Shark Bullet useless with Rashoumon. But here, I think Kisame has higher chances.