Preheat the oven to 350°F.
Grease an oven-safe pie plate and set aside.
In a medium bowl, mix together the melted butter, brown sugar, cinnamon, flour, and oats until a crumbly mixture forms.
Place the diced apples in the bottom of the greased pie plate and top with the crumbly mixture. Press it onto the apples to help cover everything evenly.
Bake for 40-50 minutes.
Let cool at least 10 minutes before serving.