Preheat the oven to 350°F.
In a mixing bowl, combine the mixed berries, lemon juice, and granulated sugar. Toss gently to coat the berries and set aside.
In another bowl, mix the rolled oats, flour, brown sugar, cinnamon, and salt.
Pour in the melted butter and stir until the mixture is crumbly and well combined.
Spread the berry mixture evenly in a greased 8x8-inch baking dish.
Sprinkle the crumble topping evenly over the berries.
Bake for 30-35 minutes, or until the topping is golden brown and the berries are bubbling.
Let it cool for about 10 minutes before serving.