In a large bowl, combine flours, oats, baking soda, and salt. Cut in butter with a pastry blender, or your fingers.
Make a well, and pour in buttermilk.
Stir together quickly, being careful not to over mix. (dough will be sticky)
Shape by hand into a flat round about 8" across
With a sharp knife, score a large, deep X across the top.
Use a pastry brush to glaze the top with buttermilk, then sprinkle with oats
Bake in center of oven for about 30 minutes.
Cool on a wire rack.
Notes
Don't have buttermilk? No problem. An easy sub is two tbsp lemon juice or vinegar to one cup of low-fat milk. Stir and allow to sit for 5 minutes, and you have a perfect buttermilk substitute.