Preheat oven to 350°F.
Gently wash the berries under cool running water, removing any stems or mushy berries. Pat dry.
Place the blueberries in an 8- or 9-inch ovenproof rimmed pan.
Melt the butter.
Add the flour, oatmeal, and brown sugar to the melted butter. Mix together until well combined.
Add the cinnamon and stir.
Drop the crumble mixture evenly over the blueberries.
Bake for 25–30 minutes.
Cool and serve.