Preheat oven to 350°F. Spray a 9-inch round pie plate with nonstick cooking spray and set aside.
To make the topping:
In a large mixing bowl, whisk together the flour, brown sugar, cinnamon, and salt. Add in the cold cubed pieces of butter and cut into the mixture with a pastry cutter (or two forks) until you have small pea sized crumbs. Stir in the rolled oats and set aside.
To make the filling:
In a separate large mixing bowl, add the berries, sugar, and flour and mix until all of the berries are well coated. Pour the mixture into the prepared pie plate and spread it around into one even layer. Sprinkle the topping evenly over the fruit.
Bake at 350°F for 40-45 minutes or until the filling is bubbly and the topping is lightly golden brown. Remove from the oven and transfer to a wire rack to cool slightly before serving.
Storage instructions: Leftover berry crisp can be stored in the fridge for up to five days. Reheat gently in the microwave or oven, if desired.