Preheat oven to 350°F (175°C). Grease a mini muffin tin or line with paper liners.
Mash the bananas in a large bowl.
Add the egg, oil, milk, vanilla, and sugar or honey. Stir until well combined.
Add the oats, flour, baking soda, baking powder, cinnamon, and salt. Stir until just combined — do not overmix.
Spoon batter into the muffin tin, filling each cup about 3/4 full.
Bake for 12–15 minutes for mini muffins (18–20 for regular), or until a toothpick comes out clean.
Let cool in the pan for 5 minutes, then transfer to a wire rack.