Preheat oven to 350°. Place rhubarb in ungreased 3 quart rectangular baking dish. Stir in 1 1/2 cups granulated sugar and 1/3 cup all-purpose flour. In medium mixing bowl combine oats, brown sugar and 3/4 cup all-purpose flour. With pastry blender, cut in butter until mixture resembles coarse crumbs. Sprinkle topping over filling. Bake in 350° oven 35 minutes or until fruit is tender and topping is golden. Serve warm with ice cream, if desired.