In a large mixing bowl, combine old fashioned rolled oats, all-purpose flour, and brown sugar. Drizzle in the melted butter and mix until the mixture resembles coarse crumbs. Preheat your oven to 350°F (175°C).

Press about two-thirds of the crumb mixture into the bottom of a greased 9x9-inch baking pan to form the crust.

In a separate bowl, toss the thawed raspberries with lemon juice and cornstarch until well-coated. Pour evenly over the pressed crust.

Sprinkle the remaining crumble mixture over the raspberry filling, ensuring even coverage across the top.

Bake in the preheated oven for about 30 minutes, or until the top is golden brown and filling is bubbly.

Let the bars cool in the pan for at least 30 minutes, then refrigerate for an additional hour before cutting.