Preheat oven to 350 degrees. Line an 8-inch square cake pan or baking dish with parchment or grease with oil.
In a medium-sized bowl, whisk together oats, flour, baking powder, baking soda, salt, and cinnamon until combined.
In a large bowl, mash the banana then stir in egg, oil, and maple syrup. Squeeze grated zucchini in a kitchen towel or paper towel to remove most of the excess water. Stir in zucchini.
Add dry ingredients and chocolate chips to the wet ingredients and mix until just combined. Pour batter into prepared pan and smooth the top to make it level. Sprinkle on some additional chocolate chips if you'd like.
Bake until lightly golden around the edges and cooked through in the center, about 18 to 25 minutes. Cool completely then cut into 9 large squares or 16 small squares. Store bars in an airtight container at room temperature for up to 2 days or in the fridge for 5 days.