Apple Crisp with Oat Topping

2 1/2 pounds medium Granny Smith (about 5 or 6), peeled, cored, and sliced 1/4 inch thick
2 to 3 tablespoons granulated sugar
3/4 teaspoon ground cinnamon
1/4 teaspoon fine salt
1/2 cup packed light brown sugar
1/2 cup uncooked rolled oats
1/3 cup all-purpose flour
4 tablespoons cold unsalted butter, cut into small pieces, plus extra for coating the dish
vanilla ice cream