Preheat oven to 350 degrees and grease a loaf pan.
Add the banana, rolled oats, almond milk, cinnamon, baking powder, and pure vanilla extract to a blender or food processor. Mix until the ingredients are combined.
Pour the mixture into the loaf pan and add the walnuts and diced apple. Sprinkle the top with brown sugar. Add an additional tablespoon of brown sugar if you like your oatmeal on the sweeter side.