Preheat your oven to 375°F (190°C). Grease a muffin tin or line with muffin liners.
In a large mixing bowl, combine the oats, flour, baking powder, baking soda, and salt.
In another bowl, whisk together the brown sugar, mashed bananas, egg, milk, vegetable oil, and vanilla extract.
Pour the wet ingredients into the dry ingredients and mix until just combined. Do not overmix.
Divide the batter evenly among the muffin cups.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.