Heat the oven to 350°F. Line a baking sheet or lightly oil it.
Peel the bananas and mash them in a bowl with a fork until mostly smooth.
Add the oats to the bowl. Stir until all the oats are coated and the mixture looks thick and sticky.
If using chocolate chips, stir them in until they are spread out.
Scoop the dough into thick mounds and place them on the baking sheet. They should look chunky, not flat.
Bake for 12–15 minutes, until the cookies look set and lightly browned on the edges.
Let the cookies cool on the pan for 5 minutes so they firm up, then move them to a plate.