In a medium bowl, mix together the oats, flour, brown sugar, granulated sugar, and cinnamon.Cut butter into small pieces and mix into the oat mixture until the butter is evenly distributed throughout the oats. Set the crumb mixture aside.
Peel and core apples. Slice into ¼-inch thin slices.
Heat butter in a 10-inch cast iron skillet over medium-low heat.
Add apples to the butter. Sprinkle the remaining ingredients over the top of the apples, then toss to coat. Cook apples over medium heat for about 3 minutes to caramelize.
Spoon the crumb topping over the apples. Bake for 30-40 minutes, or until apples are soft and the top has started to brown.Let rest for 10 minutes, then serve with ice cream.