To make the topping: In a small bowl stir together oats, brown sugar, cinnamon, and salt. Slice the room temperature butter into small pieces and place on top of the oat mixture. With your fingers, pinch the butter and oat mixture together until the butter is well-distributed and all the oats are moistened. Set aside.
To make the apple mixture: place the cut apples in a flat 10 cup baking dish (or 9" square dish or 7" x 11" Pyrex pan). Sprinkle the sugar, flour and cinnamon over the top. Stir to combine well, then spread the apples out flat and evenly in the baking dish.
Sprinkle the topping evenly over the top of the apples.
Bake for 35-40 minutes until the apples are soft and the topping is golden brown. If the topping isn't quite brown, place under a broiler for 1-2 minutes. Watch carefully so it doesn't burn. Serve warm with ice cream.
Notes
This dish can be made earlier in the day, and then reheated in a 350 degree oven for 10-15 minutes. Place under the broiler for a couple minutes if necessary to crisp up the topping.