Preheat oven to 375°F (190°C) and grease a 9x9 inch baking dish.
In a large bowl, toss the apple slices with granulated sugar and lemon juice, then spread evenly into the prepared baking dish.
In a separate medium bowl, whisk together the brown sugar, oats, flour, cinnamon, and salt.
Use a pastry cutter or fork to cut the softened butter into the dry ingredients until the mixture becomes crumbly.
Spread the oat crumble mixture evenly over the top of the apples.
Bake for 40 to 45 minutes until the fruit is bubbling and the topping is golden brown.
Remove from oven and allow to cool for 10 to 15 minutes before serving warm.