Begin by gathering all the necessary ingredients. Preheat your oven to a temperature of 350°F (175°C).
In a 9x13-inch baking dish, arrange the sliced apples evenly. In a separate bowl, combine the white sugar, one tablespoon of flour, and ground cinnamon. Sprinkle this mixture generously over the apples, ensuring even coverage. Drizzle the water evenly across the top of the apples.
Next, in a large bowl, mix together the quick-cooking oats, one cup of flour, packed brown sugar, baking powder, and baking soda. Pour in the melted butter and use a fork to mix until the mixture becomes crumbly. Distribute this crumbly topping evenly over the apple layer.
Place the dish in the preheated oven and bake for approximately 45 minutes, or until the topping turns golden brown and the apples are bubbling at the edges.