Preheat the oven to 350°F and line a baking pan with parchment paper.
In a large bowl, combine the flour, rolled oats, and granulated sugar.
Add the melted butter and stir until the mixture is evenly moistened.
Set aside approximately 1/3 to 2/3 of the mixture for the crumb topping, depending on how much topping you prefer.
Press the remaining oatmeal mixture evenly into the prepared pan to form the crust.
Bake the crust until lightly set, approximately 8–10 minutes.
While the crust bakes, combine blueberries, sugar, cornstarch, lemon juice, lemon zest, and vanilla in a bowl.
Stir gently until the blueberries are evenly coated.
Remove the crust from the oven and spread the blueberry filling evenly over the top.
Sprinkle the reserved oatmeal mixture over the blueberries.
Bake for approximately 25–30 minutes, or until the topping is lightly golden and the blueberry filling is bubbling.
Remove from the oven and cool completely on a cooling rack.
Once completely cooled, lift the bars from the pan using the parchment paper.
Cut into squares or rectangles and serve.
