Add all of the dry ingredients to a large bowl and stir until combined.
Mix in the milk, egg, mashed banana, peanut butter, maple syrup and vanilla until the ingredients are all combined.
Pour the mixture into a greased baking dish (you can use whichever size you like just keep in mind that the bigger your dish is, the thinner the oatmeal will be) and bake for 25 to 30 minutes.
Let it cool for a few minutes and then slice and serve. You can also keep the oatmeal in the fridge and serve it when you’re ready.