Gather all ingredients. Preheat the oven to 350 degrees F (175 degrees C).
Place sliced apples in a 9x13-inch baking dish. Mix white sugar, 1 tablespoon flour, and cinnamon together; sprinkle over apples. Pour water evenly over apples.
Combine oats, 1 cup flour, brown sugar, baking powder, and baking soda in a large bowl. Add melted butter and mix with a fork until crumbly; sprinkle evenly over apple mixture.
Bake in the preheated oven until apple filling is bubbling, and top is golden brown, crisp, and caramelized around the edges, about 45 minutes.
