Preheat your oven to 350°F (175°C). Grease a muffin tin or line with muffin 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, oil, vanilla extract, and egg.
Add the wet ingredients to the dry ingredients and mix 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.