Pre-heat oven to 350 degrees F.
Peel the bananas and using the back of a fork, mash them on top of a cutting board until the big lumps are gone.
Add the mashed bananas, rolled oats and peanut butter to a large bowl. Mix until combined with a spatula.
Place mounds of the mixture - about 2 Tablespoons each - onto a cookie sheet, lined with parchment paper. Press them each down a little bit with your fingers as these cookies do not spread when they bake. If it's too sticky, use the back of a wet fork.
If you're adding any chopped nuts or shredded coconut, sprinkle that on top.
Bake in the pre-heated oven for 15 minutes. Remove from the oven and let cool.
Store in an air-tight container for 3 days or freeze for up to 3 months.