Preheat your oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish with butter or nonstick cooking spray.
Prepare the filling: In a large mixing bowl, toss the peeled and sliced apples with the granulated sugar, lemon juice, and cinnamon until the apples are evenly coated.
Spread the coated apples evenly into the bottom of the prepared baking dish.
Prepare the topping: In a separate medium bowl, whisk together the rolled oats, flour, brown sugar, cinnamon, and salt.
Cut the cold, cubed butter into the dry mixture. You can use a pastry blender, two forks, or your clean hands. Work the butter in until the mixture resembles coarse, pea-sized crumbs.
Sprinkle the oat streusel topping evenly over the apples in the baking dish, covering them completely.
Bake uncovered in the preheated oven for 40 to 45 minutes, or until the topping is deep golden brown and the apple juices are bubbling around the edges.
Let the crisp cool for at least 15 minutes before serving so the juices can slightly thicken. Serve warm with vanilla ice cream!