Preheat oven to 190 degrees C. Spray a 20x30cm baking dish with cooking spray.
Mix together 1 cup of rolled oats, 20g butter and 2 tablespoons of brown sugar in a bowl; press the mixture into the bottom of the prepared baking dish.
Lightly toss together the plums, white sugar, flour, cinnamon and nutmeg in another bowl. Spread the plum mixture over the crust. Mix 1 cup of oats, 2 tablespoons of brown sugar and 20g butter in a bowl until well combined and sprinkle the crumble mixture over the top of the plums.
Bake in the preheated oven until the plum filling is bubbling and the topping is browned, about 35 minutes. Serve warm.