Add the oat flour, brown sugar, cinnamon, baking soda, baking powder, and sea salt to a mixing bowl. Stir until well combined.
Add the mashed banana, plain Greek yogurt, eggs, and vanilla extract to a bowl. Whisk together until well combined.
Add the dry ingredients to the wet ingredients, stirring until everything is well combined.
Fold in the chopped walnuts.
Transfer the batter to a muffin tin that has been lined with muffin/cupcake liners. Fill each muffin well two-thirds of the way full. Sprinkle a few chopped walnuts on top.
Bake the muffins at 350 degrees for 15-20 minutes or until a toothpick comes out clean when inserted.
Serve them warm, straight out of the oven, or enjoy them the next day.