Preheat the oven to 350 degrees F (175 degrees C). Grease a 9-inch baking dish.
Crush 1 cup raspberries with a fork. Place crushed berries in a 1-cup liquid measuring cup and add enough water to bring back to 1 full cup.
Combine sugar and cornstarch in a saucepan. Stir in raspberry-water mixture and bring to a boil. Cook, stirring occasionally, until thick, about 2 minutes. Reduce heat to a simmer. Stir in remaining 3 cups raspberries; remove from heat and allow to cool.
Meanwhile, combine oats, flour, brown sugar, and baking soda in a bowl. Cut in butter with 2 knives or a pastry blender until the mixture resembles coarse crumbs.
Press 1/2 of the crumb mixture into the bottom of the prepared baking dish. Spread the raspberry mixture on top and sprinkle with the remaining crumb mixture.
Bake in the preheated oven until the top is golden brown, 25 to 30 minutes.