Preheat the oven to 350°F (175°C).

In a mixing bowl, combine the blueberries, lemon juice, granulated sugar, and cornstarch. Toss gently to coat the blueberries evenly.

Transfer the blueberry mixture to a greased 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.

Spread the topping evenly over the blueberries 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.