Preheat the oven to 350°F (175°C).

In a mixing bowl, combine sliced plums, granulated sugar, lemon juice, and cornstarch. Toss until plums are evenly coated.

Transfer the plum mixture to a greased baking dish.

In another bowl, mix oats, flour, brown sugar, and cinnamon.

Pour melted butter over the oat mixture and stir until crumbly.

Spread the topping evenly over the plum filling in the baking dish.

Bake in the preheated oven for 30 minutes or until the topping is golden brown and the plums are bubbly.

Let cool for a few minutes before serving.