To make the crust and topping:
In a large bowl, whisk together the flour, brown sugar, oats, salt, baking powder, baking soda, and cinnamon. Using a mixer, or by hand, cut the butter pats into the dry ingredients until the mixture is crumbly, being careful to not overmix. Transfer 2 cups of the crumbs to a separate bowl, setting aside for later (those will be the crumb topping). Press the remaining crumbs into the 9×13″ pan. Bake crust for 15 minutes and remove from oven.