Grease 4 muffin molds with cooking spray. Set aside.
In a bowl, mix together the oats, flour and baking powder.
In another bowl, add the sugar and butter and beat until creamy
Add the banana and vanilla extract and beat until well combined.
Add the flour mixture and milk in banana mixture and mix until just combined.
Fold in the walnuts.
Place the mixture into the prepared muffin molds evenly.
Slide the Air Fryer Basket inside and adjust the temperature to 320 °F to preheat for 5 minutes.
After preheating, arrange the muffin molds into the Air Fryer Basket.
Slide the basket into Air Fryer and set the time for 10-12 minutes.
After cooking time is finished, remove the muffin molds from Air Fryer and place onto a wire rack to cool for about 10 minutes.
Then invert the muffins onto the wire rack to completely cool before serving.