Orochi is the user of the Hebi Hebi no Mi, Model: Yamata no Orochi.
Being a mythical zoan the fruit should have a special power, like the other MZ fruits.
Marco: regeneration
Sengoku: shockwaves
Kaido: fire breath
Devon: transform into clones
So what is going to be Orochi's unique power?
I'm not so sure about these descriptions.
I believe Sengoku's real ability is gigantification. His "shockwave attack" is not just Whitebeard's power slapped on to a Zoan, but Sengoku's advanced armament projection enhanced by his giant and enlightened Buddha body. I'm sure his obsevation haki is also enhanced in that golden form, similar to how Enel enhanced his with the Goro Goro abilitiy. It's either that or he has two abilities which are becoming a giant hybrid and using shockwaves.
Kaido can shoot fire so intense it looks like an energy blast, create clouds to change the weather and use for flight. That's definitely two distinct abilities for one Mythical Zoan. Granted, on their own even Hold'em can shoot fire and clouds are not especially effective for much. Oda may give Kaido more minor abilities like electricity attacks as the arc goes on, but I don't he'll have something major like regeneration or shape shifting which are good enough to be Paramecia.
Orochi definitely has a poison ability seeing as Sandersonia and Marigold can both spit venom, but that won't be his defining attribute. The popular guess is that he can endlessly regrow his heads, so you have to attack his cowardly main body. I don't like that there is no difference between the Beast and Hybrid form, one form may look cooler but Orochi's main body is uninvolved as that would be his weakness. Also Marco has full body regeneration and flight, outclassing Orochi. I'm going to go for a unique guess and say each of Orochi's heads can shoot out a unique element. One head spits poison, another spits fire, another spits lightning, another spits frost, another shoots air. Kinemon may cut fire but gets frozen by a separate head. Whoever defeats Orochi has to overcome all the elements.