Preheat the oven to 375 degrees F. Grease the muffin tins or line them with paper liners
Mix together the flour, baking powder, baking soda, salt, and cinnamon and set aside.
Whisk together the mashed bananas, sugar, egg, melted butter, and vanilla.
Add the flour mixture and fold in just until the dry ingredients are moistened.
Divide the batter between 12 muffin tins. The tins should be about 3/4 full.
Make the streusel topping. In a small bowl add the oats, flour, brown sugar, cinnamon, butter, and walnuts, using a fork or your fingers mix together until well combined.
Divide the streusel topping between the muffins.
Place the muffins in the oven and bake for 14 to 16 minutes. Use a toothpick check to muffins to see if they are done. Insert the pick in the muffin if it comes out clean the muffins are done.
Remove from the oven and cool in the muffin tins for 2 to 3 minutes, then remove for the tins and continue cooling on the cooling racks.