Preheat oven to 350 degrees. Grease an 8" x 8" baking pan and set aside.
Combine brown sugar, flour, baking soda, salt and oats. Cut in the softened butter with your fingers or a pastry cutter into small pea sized pieces until the mix resembles coarse crumbs.
Take half your crumble mix and press down into your greased baking pan to form a crust.
Coat your apples with one tablespoon of cinnamon sugar then spread over your crust.
Take the rest of the crumble mixture and add on top of your apples.
Bake in the oven for 35-40 minutes until the top is crisp and golden. Allow to cool completely before cutting into bars.