Prepare a 12 cup muffin pan with paper muffin cups.
In a bowl combine the brown sugar, 1/4 cup of the flour, oats, melted butter, and 1/2 tsp of the cinnamon, mix and press together with a fork until it looks crumbly, and set aside.
In another mixing bowl, combine the remaining 1 1/2 c flour, baking powder, baking soda, 1/4 tsp cinnamon, and 1/8 tsp salt. Set aside.
In another bowl combine the egg, mashed bananas, sugar, and oil, mix well then add to the flour mixture. Stir until just moistened, fold in the walnuts.
Spoon batter into each muffin cup, about 2/3 full each. Top each batter filled cup with the streusel topping, enough to cover so you cant see the batter. Bake at 350 degrees 20-25 min, or until toothpick test comes out clean.