Preheat your oven to 350°F (175°C). Lightly grease an 8x8-inch baking dish.
In a large bowl, toss blueberries with sugar, lemon juice, and cornstarch. Pour into the prepared dish and spread evenly.
In another bowl, combine oats, flour, brown sugar, cinnamon, and salt. Add the cold butter and use your fingers or a pastry cutter to work it in until the mixture is crumbly.
Sprinkle the topping evenly over the blueberries.
Bake for 30 minutes, or until the topping is golden brown and the blueberries are bubbling.
Let cool for 10 minutes before serving. Enjoy warm, maybe with a scoop of vanilla ice cream!