Preheat the oven to 350 degrees.
In a medium bowl mix together the dry ingredients, flour, sugar, oatmeal, baking powder, salt, and baking soda. Set aside.
In the mixing bowl mix together the wet ingredients, eggs, vegetable oil, milk, vanilla, and mashed banana.
Add the dry ingredients in with the wet ingredients and mix until just blended. Do not over mix.
To make the Streusel: in a small bowl mix together softened butter, oatmeal, flour, sugar, and cinnamon until very fine crumbles. Note: Larger pieces of butter will create a "crater" in the streusel top of the cooked muffins. Set aside.
Place cupcake papers into muffing pan. Add muffin batter to the cups filling them about 2/3's full.
Sprinkle about 1 to 1½ tsp of streusel on top of each muffin.
Bake for 18-20 minutes or until a toothpick inserted into the muffins comes out clean.
Cool slightly before serving. Store in an airtight container.