Preheat the oven to 350 degrees F. Grease a 9" x 13" baking dish and set aside.
Wash, peel, core, and slice the apples. Add the apple slices to the prepared baking dish and sprinkle with lemon juice (optional) and add the cinnamon. Stir or toss gently to combine.
In a mixing bowl, mix together the oats, flour, sugar, cinnamon, baking soda, baking powder, and salt.
Add melted butter to the oat topping mixture, and mix well. Sprinkle the topping evenly over the apples in the baking dish.
Bake for 40-45 minutes, or until apples are soft and the topping is a golden brown color.