Preheat oven to 350 degrees F (175 degrees C). Spray an 8-inch square baking dish with cooking spray.
Combine brown sugar, oats, flour, and butter together in a bowl until crumbly. Spread 1/2 of the oat mixture into the prepared baking dish and press firmly to form a crust. Arrange apple slices over the crust.
Mix white sugar and cinnamon together in a bowl; sprinkle over apples. Crumble remaining oat mixture over apples.
Bake in the preheated oven until topping is golden brown, 40 to 45 minutes.
