Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or grease them lightly with oil.
In a mixing bowl, combine the rolled oats, flour, baking powder, baking soda, salt, and cinnamon. Set aside.
In another bowl, whisk together the mashed bananas, brown sugar, melted butter or oil, milk, egg, and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients and mix until just combined. Be careful not to overmix; a few lumps are okay.
Gently fold in the blueberries into the batter.
Spoon the batter into the muffin cups, filling each about 2/3 full.
Optional: Sprinkle a few extra oats or a light dusting of cinnamon on top of each muffin for added texture and flavor.
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 pan for 5-10 minutes before transferring them to a wire rack to cool completely.