Preheat your oven to 350°F (175°C) and grease a muffin tin or line with muffin liners.
In a large mixing bowl, whisk together the flour, oats, baking powder, baking soda, and salt.
In another bowl, mix the mashed bananas, brown sugar, vegetable oil, vanilla extract, and egg until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Divide the batter evenly among the muffin cups.
Bake for 20-25 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.