I can see Oro winning, believe it or not. Sasuke has way more of a strain with his MS eyes than Itachi did. His Susano'o would be useless, seeing as Oro could just come up from the ground with the Kusanagi and stab him. His Amaterasu is useless because of Oro's Body Replacement technique, he can dodge any arrows thrown by entering Slithering Snake mode, etc. The only problem here is Genjutsu. Orochimaru was able to move his hands when cast under Itachi's Genjutsu, and all it takes is a flick of his fingers in order to move the Kusanagi Blade. He could either use it to extend itself at Sasuke OR at himself to have the pain wake him up from the Genjutsu, or he could enter Hydra mode to fight without looking at his eyes.
Having Zetsu's and Hashirama's cells will work in his favor. He will heal much faster than Sasuke will and have much more stamina. As well as that, any physical attacks Sasuke uses on Oro won't work. He can escape Amaterasu, if he gets sliced in half he'll be able to reattach himself, etc. Orochimaru could also use an attack on Sasuke by emerging from the ground under his Susano'o and using soft neck throw to immobilize him by using a different Juinjutsu and then proceed to either use the Kusanagi on him or the RDS.