Preheat oven to 425℉/220℃/Gas Mark 7.
In a large bowl, whisk the whole oats, all-purpose flour, granulated and brown sugar, baking powder, baking soda, salt, and cinnamon together. Set aside.
In a medium bowl, whisk the whole egg, melted unsalted butter, and vanilla together. Slowly pour into the dry mixture. The mixture will be dry but keep mixing it together until it has come together.
Fold in the mashed bananas. If the bananas are not ripe enough, the batter may still appear dry; whisk in 1-2 tbsp. whole milk until the batter no longer has any dry patches in it.
Pour the batter into greased or lined muffin tins.
Make your oat topping by mixing in a separate bowl, the whole oats, brown sugar, melted butter and cinnamon. Sprinkle on top of each of the muffins.
Bake the muffins for 5 minutes then while keeping the muffins in the oven decrease the temperature to 375℉/190℃/Gas Mark 5 and continue to bake for another 13-15 minutes.
Remove from the oven, let cool, and enjoy!