Preheat the oven to 400 degrees Fahrenheit. Grease a 12-cup muffin tin or line it with muffin liners.
In a medium bowl, combine the flour, oats, brown sugar, baking powder, baking soda, and salt. Set it aside.
In a separate, large bowl, whisk together the egg, milk, oil, and vanilla. Mix in the mashed bananas.
Add the flour mixture to the wet ingredients and mix it with a spatula until moistened. Do not overmix it. Fold in the walnuts.
Scoop the batter into the prepared muffin cups, filling them ¾ full. Bake them for 18 to 20 minutes or until the tops spring back when lightly pressed.
Let the muffins cool in the tin for 10 minutes, and then transfer them onto a wire rack to cool completely.
Serve and enjoy!
