Toss apples with white sugar and 1/2 teaspoon cinnamon in a medium bowl to coat; pour into a 9-inch square baking dish.
Mix brown sugar, oats, flour, and 1 teaspoon cinnamon in a separate bowl. Use a pastry cutter or 2 forks to mash cold butter into the oats mixture until the mixture resembles coarse crumbs; spread over the apples to the edges of the baking dish. Pat the topping gently until even.
Bake in preheated oven until golden brown and sides are bubbling, about 40 minutes.
