Preheat oven to 350°F. Grease a 9x13 inch baking dish and set aside.

In a large bowl, combine flour, brown sugar, oats, cinnamon, baking powder, and baking soda.

Pour melted butter over the dry ingredients and stir until crumbly.

Reserve one-third of the crumb mixture for topping.

Press the remaining crumb mixture firmly into the bottom of the prepared dish.

In a separate bowl, toss sliced apples with a tablespoon of flour and cinnamon.

Spread the apple mixture evenly over the crust.

Sprinkle the remaining crumb mixture over the apples.

Bake for 55-65 minutes until golden brown and bubbly.

Allow to cool before slicing and serving.