Preheat the oven to 300 degrees F (150 degrees C). Grease a 9x13-inch baking dish.
Combine apples, sugar, cinnamon, and nutmeg for filling in a bowl; transfer to the prepared baking dish.
Combine brown sugar, flour, oats, butter, and cinnamon for topping in a large bowl. Sprinkle on top of filling.
Bake in the preheated oven until golden and bubbly, about 1 hour.
