If you suspect a food sensitivity, the most important thing to do is find a food with as few protein sources as possible (its the body's reaction to proteins that most often causes food sensitivities) and find different proteins than the ones you are currently feeding.
If you are feeding chicken and oats, for example, switch to something like lamb and rice, beef and barley, salmon and peas, etc. Just be aware of the ingredient panel; it's common for foods to have multiple protein sources, and protein doesn't always come from meat. Wheat, barley, corn, peas, potatoes, etc are all common ingredients and can cause sensitivities.