Oatmeal is not allowed on the whole life challenge at the PERFORMANCE LEVEL, but is compliant at the LIFESTYLE and KICKSTART levels.
Oatmeal falls into the starches and grains category. It's a starchy carbohydrate, which is one reason that we don't allow it for Performance level. The other reason is that, like most grains, oatmeal contains compounds that are toxic to the human body and can interfere with digestion.
Gluten (in wheat, barley, and rye) is only one reason that we don't allow grains (but that doesn't apply to oatmeal except for small amounts of contamination from the factory.) Other starches are not allowed because of their impact on insulin levels.