Preheat the oven to 350 degrees F (175 degrees C). Lightly grease an 8-inch square pan.
Place brown sugar, oats, flour, and butter in a large bowl; mix until crumbly. Place 1/2 of the crumb mixture in the bottom of the prepared pan. Spread apples evenly over crumb mixture. Sprinkle with sugar and cinnamon; top with remaining crumb mixture.
Bake in the preheated oven until golden brown, 40 to 45 minutes.