Preheat the oven to 350°.
First, make the oat topping. In a medium bowl, combine the oats, flour, cinnamon, brown sugar, salt, and cold butter.
Use your hands to mix the ingredients and rub the dry ingredients into the butter until the mix is crumbly. Set aside.
Now make the apple filling. In a large bowl, stir the apples, lemon juice, cinnamon, sugar, brown sugar, and cornstarch together until the apples are well coated.
Pour the apples into an 8x8 pan or baking dish, spread them out evenly, and pour the melted butter over the apples.
Sprinkle the oat topping over the apples, and bake for 45-50 minutes or until the filling starts to bubble around the edges.
Serve warm with a scoop of ice cream or whipped cream, if desired.