In a big bowl, combine the oats, flour, brown sugar and melted butter. Mix till it becomes crumbly. (I do this step with clean hands, as it's just easier for me to get it to the consistency that it needs to be)
Place half of the mixture into the bottom of the pan. (I spray the bottom lightly with Pam)
Next, spread the chopped apples over the crumb mixture evenly.
Sprinkle the sugar and cinnamon over the apples. On top of that, add the remaining crumb mixture.
Bake for 40 to 45 minutes until golden brown.
NOTE: as I chop my apples I have a bowl of cold water with a little lemon juice to keep the apples from browning on me since I have about 5 to 6 apples to chop up depending on the size. I've always used gala apples, just because i really like the taste and they definitely hold up well in this recipe. Not mushy at all.