Preheat your oven to 350 degrees F, and grease a 9-by-13-inch baking dish.
Mix together the berries, white sugar, cornstarch and cinnamon until evenly coated and pour into the 9-by-13 dish.
Combine the oats, flour, brown sugar, salt, and butter using your hands or a pastry cutter (or both!) until the dry ingredients are well incorporated with the butter. Some lumps are fine. Add evenly over the berry mix.
Bake about 45 minutes, until the topping is browned and the filling bubbling. Cool 10-15 minutes. Serve alone or with a scoop of vanilla ice cream.