Preheat oven to 375°F/191°C. Prepare a 12-hole 3-ounce muffin pan by spraying it with non-stick spray. Set aside.
In a bowl, mix together cake mix, egg, water, oil and ripe bananas until smooth. Fold in half of the oatmeal and chocolate chips.
Scoop out muffin batter in prepared pan. Sprinkle remaining oatmeal and chocolate chips as topping.
Bake for 20-25 minutes or until a toothpick inserted in the center comes out clean.
Best served warm.