01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease lightly with cooking spray.
02 - In a large mixing bowl, whisk together flour, oats, sugar, baking soda, baking powder, salt, and cinnamon until well blended.
03 - In a separate bowl, mash bananas until smooth. Whisk in melted butter or oil, eggs, and vanilla extract until fully incorporated.
04 - Pour wet ingredients into dry ingredients. Fold gently with a spatula until just combined—do not overmix or muffins will become tough.
05 - Fold in chocolate chips and chopped nuts if using. Reserve a small handful to sprinkle on tops if desired.
06 - Divide batter evenly among the 12 muffin cups, filling each about three-quarters full.
07 - Bake for 18-22 minutes until golden brown and a toothpick inserted into the center comes out clean or with just a few moist crumbs.
08 - Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely before storing.