Peel, core, and slice your apples. I used 7 because mine were small. If you’re using large, you can use less. Apples should be thin and bite sized.
Spray crockpot with cooking spray.
Place apples in the bottom of your crockpot. Sprinkle with sugar and 1/2 teaspoon of cinnamon.
Place the cake mix in a large bowl. Add the oats and 1 teaspoon cinnamon. Stir, then sprinkle over the top of the apples.
Drop slices of the butter over the top of the cake mix.
Cover and cook on high for 4 hours. (My crockpot defaults to HIGH power for 4 hours. As you can see, it got a little dark around the edges, so if you have a "low" setting, you can try that instead.) All crockpots cook differently. Yours might take less time. Start checking at 1 1/2 hours. Once all the butter is melted and the powder is gone and the top is golden, it is done. I sometimes start with a paper towel between the lid and insert, but with this cake I like to remove it after about 2 hours so the condensation helps to dissolve the cake mix. Once you bake in your crockpot awhile you'll learn how it bakes (faster/shorter, hotter etc.) Serve with ice cream or whipped cream.