1-1/4 cups of the buttermilk in a bowl. Cover and let stand overnight at room temperature. Preheat oven to 350 degrees. Lightly grease a baking sheet and set aside. Sift together the flour, salt and baking soda. Gradually beat the flour mixture into the oat-buttermilk mixture, adding remaining buttermilk as you work, to form a soft dough. You may not need to use the full 2-1/4 cups buttermilk. Turn dough out onto a floured surface and flatten with hands or floured rolling pin so that it forms a circle about 1 inch thick and 9 to 10-inches in diameter. With a sharp knife, cut into quarters or eighths, as desired, and place on prepared baking sheet. Bake for 40 minutes, or until browned. Serve warm. Note: These are high in fiber and low in fat, provided you don't slather th with butter. Delicious with jam or marmalade. Yield: 8 servings.