Preheat oven to 375 degrees. In a medium bowl, stir together 1/2 cup flour, 1 cup sugar, oats, and salt. Using a pastry blender or two knives, cut in butter until coarse crumbs form.
In a shallow 2-quart baking dish, toss plums with remaining 2 tablespoons sugar and 1 tablespoon flour; sprinkle with oat topping. Place dish on a rimmed baking sheet. Bake until topping is golden brown, 40 to 45 minutes. Let cool 20 minutes before serving.