Preheat your oven to 400ºF and line a muffin pan with paper liners.
In a large bowl, mix the oats, flour, cinnamon, baking powder, and salt. Set aside.
In another bowl, whisk the melted butter, sugar, eggs, and vanilla until smooth. Stir in the mashed bananas.
Pour the wet mixture into the dry ingredients and stir until just combined. Don’t overmix.
Divide the batter evenly into the muffin cups—they’ll be pretty full, and that’s perfect for big, bakery-style muffin tops. (A cookie scoop works great for this!)
Bake for 15–20 minutes, or until the muffins have risen and a toothpick in the center comes out clean.