Preheat your oven to 350°F and line a 12-cup muffin pan with paper liners or lightly grease it.
In a large bowl, whisk together whole wheat flour, oats, baking powder, baking soda, cinnamon, and salt.
In a separate bowl, mix mashed bananas, honey, applesauce, Greek yogurt, eggs, vanilla, and milk until smooth.
Pour the wet ingredients into the dry ingredients and stir gently just until combined.
Fold in chopped walnuts, if using.
Divide the batter evenly among the muffin cups, filling each about ¾ full.
Bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean.
Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.