Prepare the oats by adding rolled oats to a mixing bowl; optionally pulse in a blender to slightly break them down for better binding.

Mash the bananas in a separate bowl until mostly smooth, allowing small lumps.

Combine wet ingredients by adding mashed bananas, egg, and peanut butter to the oat mixture; stir into a thick, sticky dough, adding cinnamon if using.

Adjust dough texture by checking with hands; add water one tablespoon at a time if crumbly, or sprinkle in more oats if too sticky.

Preheat oven to 350°F and line a baking tray with parchment paper; shape dough into small, flattened cookies of consistent size.

Bake for 15-20 minutes until edges are set and lightly golden, centers soft; let cool completely before serving.