Preheat the oven to 350°F (175°C). In a mixing bowl, combine the blueberries, lemon juice, granulated sugar, and flour. Toss gently to coat the blueberries.

In another bowl, mix together the oats, flour, brown sugar, and cinnamon. Pour in the melted butter and stir until the mixture is crumbly.

Spread the blueberry mixture evenly in a baking dish. Sprinkle the oat topping over the blueberries. Bake in the preheated oven for 30 minutes or until the topping is golden brown and the blueberries are bubbling.

Let cool slightly before serving. Enjoy warm, optionally with vanilla ice cream.