Forget oil. Oil only takes care of the mites that are ON the snake. If you see one mite, you've got a thousand, all hiding in the cage.
I *HIGHLY* recommend the Black Knight treatment suggested by Pro Exotics. I have used it on my own snakes as well as other peoples', with great success.
Note: When Robyn says you need to over-react, he's not kidding. One of my friends who got mites from a pet-store purchase didn't break down his cages or do the follow-up treatments. He got mites back with a freaking VENGEANCE.
http://www.proexotics.com/FAQ_answers_get_rid_of_mites.html
Pro Exotics Black Knight Page
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2.0 Ball Pythons
2.0 Bettas
1.1 Pekingeses
1.4 Rats (for breeding)
1.0 Mouse (for scenting rat pinkies)