In a medium mixing bowl, combine the oatmeal, flour, brown sugar, and salt.
Melt the butter and pour over the oatmeal mixture.
Add the vanilla extract and stir until everything is well combined and lumpy.
Remove 3/4 cup of the oatmeal mixture and set aside.
Press the remaining oatmeal mixture into an 8x8 baking pan that has been lined with aluminum foil and sprayed with non-stick spray.
In a medium bowl, add the blueberries, granulated sugar, lemon juice, zest, and corn starch.
Stir gently to coat all the blueberries.
Pour over the prepared crust.
Sprinkle the remaining oatmeal mixture evenly over the top of the blueberries.
Bake in a preheated 375-degree oven for 40-45 minutes until bubbly and the topping is browned.