Pre-heat oven to 170°C / 338°F.
In a mixing bowl, mash bananas with a fork. Then, add in all other wet ingredients and mix until combined. Set aside.
Put quick oats, baking soda, salt and cinnamon in a food processor or high-power blender and blitz until combined and the texture becomes powdery like flour. Add mixture to mixing bowl and mix until combined and no lumps remain.
Fold in chopped walnuts.
Grease your muffin tin, and pour the batter in. Fill each cup to about 3/4 full.
Bake for 25 minutes or until an inserted toothpick comes out clean.
Transfer to cooling rack and let them cool down completely before storing in an air-tight container.