Preheat your oven to 350 F. Grease a 9x13” baking dish.
In a microwave-safe bowl, combine the oats and milk. Microwave on HIGH for 5 to 6 minutes, stopping to stir once about halfway through. Let the oatmeal sit on the counter to thicken for 10 minutes, but no longer.
Once it’s thickened, stir the cake mix into the oatmeal mixture, then add the salt, eggs, vanilla extract, and melted butter. Pour batter into the prepared baking dish. Drop the peaches and blackberries on top of the batter. They’ll sink a little but it’s fine.
Bake for 30 to 35 minutes, until the edges are set and the center is jiggly but not liquidy. Let cool until warm but no longer piping hot, then spoon into bowls and eat warm.