Line your muffin tin with wrappers and preheat your oven to 375°F. This recipe yields about 12 oatmeal cups.
In a large bowl, mix oats, walnuts, brown sugar, baking powder, cinnamon, salt, and berries. Gently fold in the banana slices until evenly distributed. Set aside.
In a separate bowl, whisk together milk, egg, olive oil, and vanilla. Pour over the oat mixture, stir, let rest 5–10 minutes, then mix again.
Use a ladle to transfer the mixture into each muffin cup, filling about 3/4 full. Spoon extra liquid from the bottom of the bowl over each one.
Bake for 35–45 minutes until the tops are golden and set. Let cool slightly on a wire rack before serving or storing.
Notes
Great for freezing and reheating! Swap berries or nuts as desired for flavor variety.