Preheat oven to 375° F.
Line sheet pan with parchment paper (or line pan with foil and spray with nonstick spray).
Cut butter into 1-inch cubes; set aside in refrigerator to keep cold.
Sift together flour, 1/2 cup sugar, baking powder, baking soda, salt. Stir in oats.
Using pastry blender, hands, or two forks, gently cut butter peices into flour mixture until the mixture resembles coarse crumbs.
Add buttermilk to mixture; start with 1 cup then add more as needed for batter to come together. Mix gently using hands or wooden spoon; do not overmix.
If you'd like to add a filling to scones (such as raisins, chocolate chips, dried fruit, blueberries, cranberries, or chopped nuts), gently stir in 1/2 cup of the filling.
Using hands or an ice cream scoop with handle release, shape scone batter into loose balls, 2 1/2 to 3 inches in diameter.
Place on pan 2" apart.
Mix remaining 1/2 cups sugar with cinnamon. Sprinkle on top of scones.
Bake for 15-20 minutes on middle rack of oven until scones are golden brown on top and done in center.
Allow to cool slightly on rack; serve warm.