Preheat the oven to 350 degrees. Prep the muffin tin (12 cups) with cupcake or muffin liners. Set aside.
In a large bowl, whisk together the oatmeal, flour, sugar, baking powder baking soda, cinnamon, nutmeg, and salt. Set aside.
In a smaller bowl, whisk together the egg, oil, and vanilla, then add them all to the dry ingredients. Add in the mashed banana. Gently fold it all together until just combined.
Scoop the banana batter into the prepared 12 cupcake/muffin cups.
In another small bowl, stir together the oatmeal, cinnamon, brown sugar, and melted butter. Top the muffins with the oatmeal crumble, then bake for 18-20 minutes. Remove and let cool on a wire rack until ready to eat.