Preheat oven to 350 degrees F (175 degrees C).

Mix apples, white sugar, 1 tablespoon flour, cinnamon, and salt together in a bowl; spread into an 8-inch square baking dish.

Combine oats, 3/4 cup flour, brown sugar, baking soda, and baking powder together in a separate bowl; add butter and mix until crumbly. Crumble topping evenly over apple filling using your hands.

Bake in the preheated oven until topping is lightly browned, 30 to 40 minutes.