Gather all ingredients. Preheat the oven to 400 degrees F (205 degrees C). 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.
