Preheat oven to 375°F (190°C) and grease baking dish.
Toss plums with sugar, flour, vanilla, cinnamon, nutmeg, and salt.
Spread plum mixture evenly in baking dish.
Combine oats, flour, sugar, cinnamon, and salt in bowl.
Cut in cold butter until crumbly texture forms.
Sprinkle topping evenly over plums.
Bake 35–40 minutes until golden and bubbling.
Cool for 10 minutes before serving.
Serve warm with ice cream or whipped cream.