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.

Transfer the blueberry mixture to a baking dish.

In another bowl, mix together the oats, flour, brown sugar, and cinnamon.

Pour the melted butter over the oat mixture and stir until combined.

Sprinkle the topping evenly over the blueberry filling in the baking dish.

Bake in the preheated oven for 30 minutes, or until the topping is golden brown and the blueberries are bubbling.

Remove from the oven and let cool slightly before serving.