David, I was told to not put so much as a lotion on my skin before my radiation treatments. I can't imagine the radiation onc would want you to coat your mouth with oil before the treatment. You need to ask the doctors about this, as you don't want anything to interfere with the efficiency of the radiation.
Best of luck,
Minnie

SCC Left Mandible. Jaw replaced with bone from leg. Neck disection, 37 radiation treatments. Recurrence 8-28-07, stage 2, tongue. One third of tongue removed 10-4-07. 5-23-08 chemo started for tumor behind swallowing passage, Our good friend and much loved OCF member Minnie has been lost to the disease (RIP 10-29-08). We will all miss her greatly.