Blend together chopped rhubarb, strawberry, ½ cup sugar, cornstarch, and ¼ cup flour. Pour into a buttered 10-12 inch skillet, and heat over medium for 10 minutes, stirring occasionally, until thickened. Set aside.
Stir 1 cup flour, rolled oats, sugar, cinnamon and water together, then cut in butter with a pastry cutter or two forks until you have a crumbly, slightly moist texture.
Sprinkle topping to cover your filling, then bake at 350 degrees for 35-40 minutes.