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 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. Add melted butter or oil, eggs, and vanilla extract. Mix until fully incorporated.
04 - Pour wet ingredients into dry ingredients. Gently fold together with a spatula just until combined—do not overmix as this will result in tough muffins.
05 - Fold in chocolate chips and optional chopped nuts until evenly distributed throughout batter.
06 - Divide batter evenly among the 12 muffin cups, filling each about three-quarters full.
07 - Bake for 18–22 minutes until a toothpick inserted into the center comes out clean or with moist crumbs.
08 - Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.