Mash the bananas thoroughly with a fork in a mixing bowl.
Add the remaining ingredients and mix until you have a soft, malleable dough. If needed, add a little more oats or milk to adjust the texture.
Let the mixture rest for 10 minutes to allow the oats to soften.
Preheat the oven to 180°C and line a baking tray with parchment paper.
Roll the mixture into equal-sized balls (about 12), then gently flatten them between your palms.
Place the cookies on the prepared baking tray.
Bake in the middle of the oven for 15–20 minutes, until set and lightly golden.
Alternatively, cook in an air fryer at 180°C for about 15 minutes, turning once halfway through.
Transfer to a wire rack and allow to cool before serving.
Store in an airtight container for a couple of days, or refrigerate for longer keeping.