Peel, core and slice the apples. Mix with the blackberries. Add sugar and water to fruit and stir to combine. Place fruit in a 1 quart baking dish.
Crumble: Cut butter into flour, add oat flakes and brown sugar and continue cutting together until mixture becomes crumbly. Sprinkle over fruit, packing down lightly. Bake in a preheated 400°F oven for 15 minutes. Reduce heat to 375°F and continue baking for another 15-20 minutes until topping is crunchy brown and fruit has released its juices. Serve hot with cream, whipped cream or ice cream. Makes 6-8 servings.