Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the rolled oats, graham cracker crumbs, brown sugar, and melted butter. Stir until the mixture is evenly coated and resembles coarse crumbs.
Press about two-thirds of this crumb mixture firmly into the bottom of a greased baking pan to form the crust layer. Use the back of a spoon or your fingers to create an even, compact surface.
Spread the sliced strawberries evenly over the crust. In a separate bowl, toss the strawberries with granulated sugar, cornstarch, and vanilla extract until the berries are well coated and juices start to form.
Spoon the coated strawberries over the crust, distributing them evenly across the pan.
Sprinkle the remaining crumble mixture evenly over the strawberries, ensuring they are mostly covered for a crunchy topping.
Bake in the preheated oven for about 30 minutes, or until the crumble is golden brown and bubbling around the edges. The strawberries will release their juices and meld with the crumble as it bakes.
Remove the pan from the oven and let it cool for at least 15 minutes. The bars will firm up as they sit, making them easier to cut and serve.
Cut the bars into squares or rectangles, revealing the layered, berry-filled interior and crumbly topping. Serve them slightly warm or at room temperature for the best flavor and texture.
