Put fruit in a large ovenproof skillet with 1/2c of the water and the sugar. Bring to a boil, reduce heat and cook, covered until fruit is desired tenderness(cooking times will vary with types of fruit).
Combine the cornstarch and the remaining 1/4c of water and whisk til smooth. Add to the fruit mixture along with the butter and stir until juices are thickened and butter is melted. Remove from heat. Preheat broiler.
Combine the oats, flour and sugar. Cut in the butter until the mixture is well combined and crumbly. Sprinkle the topping over the filling and place under the broiler for 2-4min. until golden brown.