Prepare oatmeal, banana, peanut butter, and walnuts.
Mash the banana with a fork.
In a large bowl, add oatmeal, mashed banana, peanut butter, and chopped walnuts. Mix well.
Shape the dough into bite-sized balls.
Line a baking sheet with parchment paper and place the cookies on it.
Bake in a preheated oven at 180°C (356°F) for 15-18 minutes.
Remove from the oven and let them cool completely before eating.