Preheat an oven to 400 degrees F (200 degrees C). Grease 12 muffin cups, or line with paper muffin liners. Whisk together 1 1/2 cups oatmeal, the flour, baking powder, baking soda, and 1/8 teaspoon of cinnamon in a mixing bowl; set aside.
Whisk 1/2 cup brown sugar, egg white, applesauce, and milk together in a mixing bowl until smooth. Stir in the flour mixture until evenly moistened; it's ok if small, dry lumps remain. Pour the batter into the prepared muffin tin. Stir the 1/4 cup oatmeal, 1/4 cup brown sugar, 1 tablespoon cinnamon, and the melted butter in a small bowl; sprinkle this mixture over the muffins.
Bake in the preheated oven until golden and the tops spring back when lightly pressed, about 20 minutes. Cool at least 10 minutes before serving.