Preheat over to 400 deg F. Grease a 12-cup muffin pan or line with paper liners. Stir topping ingredients together in a bowl, set aside. Whisk together oats, flour, brown sugar, cinnamon, baking powder, baking soda, and salt together in large bowl. Stir in applesauce, milk, oil, egg, and vanilla extract together in a bowl. Stir applesauce mixture into flour mixture just until all ingredients are moistened; stir in apple. Spoon apple mixture into prepared muffin cups about 2/3 full; sprinkle oat topping mixture evenly over each muffin. Bake in the preheated oven until a toothpick inserted near the center comes out clean, about 15 minutes.