Preheat the oven to 350°F (175°C). Grease or line a 12-cup muffin tin with paper liners.
In a large bowl, mash the ripe bananas until smooth.
Add the egg, milk, melted butter (or oil), and brown sugar to the bananas. Stir until well combined.
In a separate bowl, whisk together the rolled oats, flour, baking powder, baking soda, and salt.
Gradually fold the dry ingredients into the wet mixture until just combined. Do not overmix.
If using, fold in chocolate chips, chopped nuts, or other add-ins.
Spoon the batter evenly into the prepared muffin tin, filling each cup about 3/4 full.
Bake for 18–22 minutes, or until a toothpick inserted in the center comes out clean.
Allow muffins to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.