Gather all ingredients. Preheat the oven to 400 degrees F. Line a 12-cup muffin tin with paper liners.

Combine flour, oats, sugar, baking powder, soda, and salt in a medium bowl; set aside.

Beat egg lightly in a large bowl. Whisk in milk, oil, and vanilla.

Stir in mashed bananas.

Add the flour mixture and stir until just combined.

Spoon batter into the prepared muffin cups, filling each 3/4 full.

Bake in the preheated oven until tops spring back when lightly pressed,  18 to  20 minutes.