Preheat oven to 375°F (190°C).
In a large mixing bowl, combine the sliced damson plums, white sugar, cornstarch, cinnamon, and nutmeg. Stir until the plums are evenly coated.
Transfer the plum mixture to a 9x13 inch baking dish.
In the same mixing bowl, combine the flour, oats, brown sugar, and butter. Use your hands to mix the ingredients together until a crumbly texture forms.
Sprinkle the crumbly mixture evenly over the plum mixture in the baking dish.
Bake for 35-40 minutes or until the top is golden brown and the filling is bubbly.
Serve warm with vanilla ice cream or whipped cream, if desired. Enjoy!