Let's see...
When Kakashi originally confronted Oro, he was literally shaking in his boots. Naturally, this was a long time ago, at this point, I would assume the fear has reduced, but I highly doubt he can FULLY shake that fear. This gives Oro an immediate psychological advantage. Beyond that, Oro probably knows more about sharingan than Kakashi does, so he won't be taken by surprise.
Orochimaru is also a wind element user - the perfect counter to Kakashi's natural lightning chakra. Also, Orochimaru is known for constantly using mud clones and shedding his skin as deceptions, making it tantimount suicide to use a jutsu like kamui on him (which would likely be the ONLY reasonable way Kakashi could win).
Orochimaru wouldn't even have to go into his true form to win. If he did that, Kakashi has no chance. He would not know about the paralyzing toxin ALL of the snakes have and Kakashi would quickly fall. And with all those snakes making up Oro's body attacking at once, it makes landing a kamui highly unlikely.
Then let's talk about summons... Kakashi has no answer for Orochimaru's Hydra Jutsu or an edo tensei of Hashi and Tobirama. Orochimaru would win this confrontation nearly 100% of the time.