Preheat oven to 375°F (190°C). Grease a 9-inch baking dish.
In a large bowl, combine sliced apples, lemon juice, cinnamon, and 1/2 cup brown sugar. Toss to coat evenly.
Transfer apple mixture to the prepared baking dish.
In a separate bowl, combine rolled oats, flour, 1/2 cup brown sugar, and salt.
Cut in the cold butter using a pastry blender or your fingers until the mixture resembles coarse crumbs.
Sprinkle oat topping evenly over the apples.
Bake for 30–35 minutes, or until the topping is golden brown and the apples are tender.
Let cool slightly before serving.