Preheat oven to 400°F/200°C and prepare a standard muffin pan with liners.
In a large bowl, whisk together the flour, oats, brown sugar, baking powder, and salt.
In a small bowl, whisk together the butter, sour cream, Greek yogurt, and egg, until well combined. Use a rubber spatula to stir this mixture into to the dry ingredients until just moistened. Gently fold in the chopped peaches. Scoop the thick batter evenly into the muffin pan.
In a small bowl, combine the cinnamon and sugar for the topping. Sprinkle it over the tops of the muffins.
Bake muffins for 16-18 minutes, or until a toothpick inserted into the center of a center muffin comes out clean. Let cool on a wire rack.