Preheat the oven to 350 degrees F. Grease a 9-by-13-inch baking dish with cooking spray or butter.
Put the berries in the prepared baking dish. Evenly distribute the cake mix over the berries. Sprinkle with the walnuts and oats. Evenly drizzle the butter over the entire dish.
Bake until firm and golden brown, about 55 minutes. Let sit 5 minutes. Serve with ice cream or whipped cream.