Preheat oven to 350℉. Spray a baking sheet with nonstick spray.
On a plate, mash the banana with a fork.
In a mixing bowl, combine the oatmeal and banana, stirring until well mixed.
Use a cookie scoop or spoon to form into twelve rounds of about 1½ inches each. Flatten slightly if you desire more of a cookie shape. Keep in a round ball if you want a cookie that is softer in the center. (Cookies do not spread when baking.)
Bake for 15 minutes or until cookies are firm but not overdone. Allow to cool, then enjoy.