Preheat oven to 350 degrees F.
In a large bowl, combine apples, white sugar, 1/2 cup brown sugar, flour, cinnamon and water. Mix well until apples are completely coated and ingredients turn into a syrup like consistency. Stir in the two tablespoons of cold butter that have been cut into small chunks. Pour mixture into a 13x9 inch pan that has been lightly sprayed with cooking oil.
In a separate bowl, combine oats, flour, 1 cup brown sugar and melted butter to create the topping. Once mixed well, sprinkle over the apple mixture in your baking dish.
Bake for approximately 45 minutes. Remove from oven and allow to cool slightly prior to serving. Store tightly covered.