Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners.
In a large bowl, mix together the oats, flour, baking powder, baking soda, and salt.
In another bowl, combine the mashed bananas, brown sugar, vegetable oil, vanilla extract, and beaten egg.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Spoon the batter into the prepared muffin tin, filling each cup about ¾ full.
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.