Preheat your oven to 350°F (175°C).

In a large mixing bowl, gently toss blueberries with granulated sugar, lemon juice, and cornstarch.

In another bowl, combine oats, flour, brown sugar, and cinnamon. Cut in cold butter until mixture resembles coarse crumbs.

Pour the blueberry mixture into an ungreased 8x8-inch baking dish and spread the crisp topping over the blueberries.

Bake in the preheated oven until the topping is golden brown and the blueberries are bubbly.