Well Itachi has already defeated him twice.
One when Orochimaru was trying to take his body, but failed. I think he was 'defeated' in that.
The second when Sasuke was fighting Itachi and out pops Orochimaru. Itachi sealed him up, so that's also a defeat.
Sasuke as well has defeated him. In the fight - if you can call it that, Sasuke killed Orochimaru when he was trying to take over his body.
In a sense the Third Hokage defeated him as well by robbing Orochimaru of his justu.
Also Jiraya, Tsunade, Shizune, and Naruto defeated Orochimaru and Kabuto when Orochimaru was trying to get his arms healed by Tsunade. It was a close battle, but in the end Orochimaru and Kabuto fled. Fleeing = Giving up = Defeat.
Some characters also had potential to defeat Orochimaru. Deidara wanted to defeat him for an unknown reason I believe, Naruto wanted to defeat him to get Sasuke back, Suigetsu wanted to defeat Orochimaru I imagine for keeping him in that water tank and experimenting on him, and perhaps Kabuto even wanted to defeat him someday to prove his strength or something. I'm sure a couple other characters wanted to defeat him too. ^-^
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