Preheat oven to 375F. Lightly grease a 9X13 inch baking dish.
In a large mixing bowl combine rhubarb, white sugar, and 3 tablespoons flour. Stir well and spread evenly into baking dish. Set aside.
In a large mixing bowl combine brown sugar, oats and 1 ½ cups flour. Stir well then cut in butter until mixture is crumbly. Sprinkle mixture over rhubarb layer.
Bake for 40 minutes. Serve hot or cold.