1Preheat oven to 375 degrees F (190 degrees C). Lightly grease a 9x13 inch baking dish.
2In a large mixing bowl combine rhubarb, white sugar, and 3 tablespoons flour. Stir well and spread evenly into baking dish. Set aside.
3In a large mixing bowl combine brown sugar, oats, and 1 1/2 cups flour. Stir well then cut in butter or margarine until mixture is crumbly. Sprinkle mixture over rhubarb layer.
4Bake in preheated oven for 40 minutes. Serve hot or cold.