Combine dry ingredients. Cut in butter. Remove 1 tablespoon beaten egg for topping; add buttermilk to rest of egg. Add to dry ingredients and stir briefly to make a sticky dough. Turn onto floured surface and knead lightly a few times until smooth. Pat out to ¾-inch thickness and cut into 2-½-inch triangles. Brush tops with reserved egg and sprinkle with oats. Bake at 425 degrees F for 12 to 14 minutes, or until golden.