Preheat the oven to 350 degrees F and lightly grease an 8-inch square baking dish.
In a large bowl, toss the apple slices with granulated sugar, cinnamon, and lemon juice until evenly coated.
Spread the apple mixture into the prepared baking dish in an even layer.
In a medium bowl, stir together the rolled oats, flour, brown sugar, and salt.
Pour the melted butter over the oat mixture and stir until the topping forms small clumps.
Sprinkle the oat topping evenly over the apples.
Bake for 35 to 40 minutes until the topping turns golden and the apples bubble around the edges.
Remove from the oven and let it rest for 10 minutes before serving.