Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners.
In a large bowl, combine the oats, flour, baking powder, baking soda, and salt.
In another bowl, mix the mashed bananas, brown sugar, vegetable oil, vanilla extract, and egg until well combined.
Add the wet ingredients to the dry ingredients and stir until just combined. Do not overmix.
Divide the batter evenly among the muffin cups.
Bake for 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.