In a small pot, add your milk or water and bring to a light simmer. Add the oats and salt and cook as usual. Cooking time will depend on the type of oats you're using (5-30 minutes).
Once most of the liquid is absorbed and oats are almost ready, add mashed banana and let it melt into the oatmeal. This should take a minute or two.
In the meanwhile, whip up the egg whites until light and fluffy. Gently fold into the oatmeal with a spatula. Cook on low for 2-3 minutes, carefully stirring a few times.
Take oatmeal off the heat and add cinnamon if you like. Cover with a lid and let sit for 3-5 minutes.
Add your favorite toppings and enjoy! For a chocolate flavor: add a few squares of chocolate and let it melt into the oatmeal.