Preheat the oven to 350°F.
In a large bowl, toss the apples with the lemon juice, sugar, and cinnamon until evenly coated.
Place the sliced apples into a deep 8×11-inch baking dish.
In a separate bowl, stir together the oats, flour, brown sugar, granulated sugar, and melted butter until crumbly.
Spoon the topping evenly over the apples.
Bake for about 50–60 minutes, or until the apples are tender and the topping is golden brown.
Let cool slightly before serving.