Preheat oven to 375 degrees. Line a standard-size 12-count muffin pan with paper liners.
In a medium bowl, stir together the flour, rolled oats, sugar, baking soda, baking powder, cinnamon, and salt. In another small bowl, mash your bananas with a fork.
In a large bowl, combine the mashed banana, vegetable oil, milk, egg, and vanilla extract. Now add the dry mixture to the wet mixture and stir just until combined.
Fill the muffin tin just shy of the top with the batter.
in a medium bowl, make the topping. Stir together the flour, rolled oats, brown sugar, and cinnamon. Pour in the butter and stir with a fork until the mixture forms crumbs.
Sprinkle the crumb mixture evenly over the top of the muffins and gently press down, so it adheres. Bake for 18-20 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs.
Let the muffins cool in the pan for 5-10 minutes before moving the muffins to a wire rack to cool.