Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius) and grease an 8x8 inch baking dish.
In a large mixing bowl, combine the rolled oats, baking powder, ground cinnamon, and salt.
In a separate bowl, whisk together the mashed bananas, milk, egg, maple syrup, and vanilla extract until uniform.
Pour the liquid mixture over the dry oat mixture and stir until fully integrated.
Fold the chopped walnuts into the batter.
Pour the mixture into the prepared baking dish and level the surface with a spatula.
Bake for 35 minutes until the oats have absorbed the liquid and the top is golden brown.
Remove from oven and let stand for 5 minutes before slicing and serving.