Preheat oven to 400 degrees F, and line a muffin tin with paper liners.
In a bowl combine flour, oats, brown sugar, baking powder and salt.
In another bowl, whisk together butter, sour cream and egg. Pour this mixture over the dry ingredients mixture and stir well to combine. Add peaches and stir to combine.
If you are making the cinnamon sugar, combine sugar and cinnamon in a small bowl and stir nicely.
Pour the batter evenly into your muffin cups and sprinkle cinnamon sugar on top (optional).
Bake for 16-18 minutes or until a tester inserted in the middle comes out clean.
Remove from oven, let it cool and the muffins are ready to serve!