Preheat the oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners or spray with oil.
In a large bowl, mash the ripe bananas until mostly smooth; a few small chunks are fine.
Whisk in eggs (or flax eggs), oil or applesauce, honey or maple syrup, vanilla, and yogurt until combined.
In a separate bowl, stir together whole wheat flour, rolled oats, baking powder, baking soda, cinnamon, and salt.
Pour the dry ingredients into the wet ingredients and fold gently until just combined; avoid overmixing.
Fold in walnuts or chocolate chips if using, reserving a few for the tops.
Spoon batter evenly into the prepared muffin cups, filling each about three-quarters full.
Top muffins with reserved oats or mix-ins for texture and appearance.
Bake for 18–22 minutes, checking at 18 minutes; tops should be golden and a toothpick inserted in the center should come out clean.
Cool in the tin for 5 minutes, then transfer muffins to a wire rack to cool completely before storing or serving.