Prep. Preheat oven to 425ºF. Line muffin tin with paper lines and set aside.
Make Oat Flour. Add 2 1/4 cups old-fashioned oats to a blender or food processor and pulse until finely ground resulting in about 1 1/2 cups oat flour. Transfer the oat flour to a large mixing bowl.
Whisk dry ingredients. Whisk in the 1/2 cup old-fashioned oats, baking powder and salt into the oat flour.
Mix wet ingredients. In a separate medium bowl, beat bananas, eggs, vanilla and brown sugar until well combined. Mix the oat flour mixture into the banana mixture until thoroughly mixed. Mix in melted butter until the batter is well combined. Stir in the pecans, if using.
Bake and serve. Scoop batter into lined muffin cups, filling them about 3/4 full. Bake for about 20 minutes until golden brown or until the center springs back when touched. Remove from the oven and cool before serving.