Preheat the oven to 400 degrees. Mix the flour, oatmeal, baking powder, baking soda, cinnamon and sugar together in a large bowl. Mix in the egg and buttermilk and stir until mixture just comes together. Stir in the apple.
Grease a muffin pan with cooking spray and fill each cup 2/3 full with the muffin mixture.Sprinkle the top of each muffin with the brown sugar. Bake for 15-20 minutes until golden brown.