How can oatmeal prevent cancer?

Don't be so sure. Internet junk sites are full of claims about "superfoods" with supposed near-magical powers. It should be obvious that these are entertainment and salesmanship. Oatmeal has been represented as blocking absorption of cholesterol to help control atherosclerosis, and as fiber for preventing colon cancer. Pretty much everything has "antioxidants" if you look hard enough. Eat it if you like it.