Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or grease it lightly.
In a large bowl, whisk together the oats, flour, baking powder, baking soda, and salt.
In another bowl, mix the brown sugar, mashed bananas, egg, milk, vegetable oil, and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Gently fold in the blueberries.
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, then transfer to a wire rack to cool completely.