Preheat oven to 350°F (175°C). Lightly grease a 13"x9" baking dish.
Berries: Place berries in a large pot. In a separate bowl, stir together sugar and corn starch (this helps to eliminate any clumps in the corn starch). Stir this mixture into the pot with the berries and heat on the stove over medium heat, stirring occasionally, until mixture is thickened.
Crunch: In a large bowl combine oats, flour and brown sugar. Stir until well combined, breaking up any clumps of brown sugar as needed. Using a pastry blender, two knives or (my favorite) a cheese grater, cut butter into oat mixture until it resembles coarse crumbs (roughly pea sized).
Assemble: Sprinkle half of the oat mixture into the bottom of your prepared baking dish. Use your fingers to pat it down into an even layer. Pour the cooked berry mixture over the top and spread out evenly. Top with the remaining half of the oat mixture.
Bake: Place baking dish into center of your preheated oven. Bake for 40-45 minutes or until crust is golden and berries are bubbling.