Preheat oven to 350°F. Grease a 9 ½ inch deep dish pie plate or similar sized baking dish. Set aside.
Topping:
In a large bowl, combine oats, flour, sugars, salt, and cinnamon. Add melted butter and stir until well combined. Transfer to the freezer for a few minutes while you assemble the fruit filling.
Filling:
In a large bowl, combine all filling ingredients. Stir gently until well combined. Dump into prepared baking dish. If using frozen berries, some of the flour/sugar might have settled on the bottom of your mixing bowl. Add it to the baking dish with the berries.
Sprinkle the topping over the berries (pressing some of it together with your fingers to form clumps). Bake for 40-50 minutes until the fruit is bubbling and juicy and the topping is light golden brown. Cool slightly, serve warm with vanilla ice cream.
Notes
*Keep the berries frozen. Do not thaw before assembly.USING FRESH BERRIES: If you'd prefer to use fresh berries, use the same amount (6 cups). I recommend adding 2 tablespoons of orange juice, apple juice, or water to the fruit filling mixture. The extra liquid helps to create a sauce while the berries are cooking. The frozen berries have enough extra liquid on their own.