Preheat oven to 350°.
Combine first 4 ingredients in a bowl; cut in margarine with a pastry blender or 2 knives until mixture resembles coarse meal. Set aside.
Combine peaches and blueberries in a 2-quart casserole, and sprinkle 1/4 cup sugar evenly over fruit. Crumble oat mixture evenly over fruit. Bake at 350° for 45 minutes or until cobbler is bubbly. Top cobbler with frozen yogurt, if desired.