Preheat oven to 450°.
Combine first 5 ingredients in a medium bowl; cut in 1/4 cup margarine with a pastry blender or 2 knives until mixture resembles coarse meal. Add buttermilk, stirring just until flour mixture is moist. Turn dough out onto a baking sheet coated with cooking spray. Pat dough into an 8-inch circle; set aside.
Combine oats, brown sugar, 1 tablespoon margarine, and 1 tablespoon flour, forming a streusel. Gently pat oats mixture into surface of dough. Cut dough into 12 wedges (do not separate wedges). Bake at 450° for 15 minutes or until lightly browned. Serve warm.