Preheat oven to 400°F. Coat a 12-cup muffin pan with cooking spray.

Combine 2 tablespoons oat flour, 2 tablespoons brown sugar, butter, oats and ½ teaspoon cinnamon in a medium bowl. Mix with a fork until coarsely crumbled.

Whisk the remaining ¾ cup oat flour, ½ teaspoon cinnamon, gluten-free all-purpose flour, baking powder, baking soda and salt in a medium bowl. Whisk the remaining ½ cup brown sugar and melted butter in a large bowl. Add buttermilk, egg and vanilla and whisk to combine. Add the flour mixture to the wet ingredients and stir just until combined. Fold in peaches. Divide the batter among the prepared muffin cups. Sprinkle with the topping.

Bake the muffins until golden brown and a toothpick inserted in the center comes out clean, 18 to 20 minutes. Let cool in the pan for 10 minutes. Transfer to a wire rack and cool at least 5 minutes more before serving.