Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners.
In a large bowl, whisk together the oats, flour, brown sugar, baking powder, baking soda, and salt.
In another bowl, mix the mashed bananas, egg, melted butter, and vanilla extract.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Divide the batter evenly among the muffin cups.
Bake for 20 minutes or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.