Preheat and prep. Heat the oven to 350°F (175°C).
Line a 12-cup muffin tin with paper liners or lightly grease with oil.
Mash the bananas. In a large bowl, mash the bananas with a fork until mostly smooth. A few small lumps are fine.
Whisk the wet ingredients. Add eggs, Greek yogurt, oil, maple syrup, and vanilla. Whisk until the mixture looks creamy and well combined.
Add the dry ingredients. Sprinkle oats, flour, protein powder, cinnamon, baking powder, baking soda, and salt over the wet ingredients.
Stir gently with a spatula until just combined. Do not overmix or the muffins can turn dense.
Fold in extras. If using chocolate chips, nuts, or berries, fold them in now. Keep the batter thick but scoopable.
Fill the muffin cups. Divide the batter evenly among the 12 cups. They should be about 3/4 full.
Bake. Bake for 16–20 minutes, or until the tops are set and a toothpick inserted in the center comes out clean or with a few moist crumbs.
Cool. Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
This helps them set and prevents soggy bottoms.
Enjoy. Serve warm or at room temperature. They pair well with coffee, tea, or a smear of almond butter.