Preheat your oven to 350°F (175°C).
In a 9x13-inch baking dish, mix sliced apples with white sugar, 1 tablespoon of flour, and cinnamon. Pour water evenly over the mixture.
In a medium bowl, combine oats, 1 cup flour, brown sugar, baking powder, and baking soda. Stir in the melted butter and chopped nuts until crumbly.
Sprinkle the oat mixture evenly over the apples.
Bake for about 45 minutes or until the topping is golden and the apples are tender.