Preheat the oven to 400°F. Grease a 9x13 pan with butter. Place the cranberries and apples in a large bowl and sprinkle with sugar, cinnamon, nutmeg, salt, and cornstarch. Stir to coat the fruit and set aside while assembling the topping.
Stir together the oats, flour, brown sugar, and butter. Pour the fruit into the greased pan and sprinkle the oat mixture on top of the fruit.
Bake for 24-28 minutes, until the crust has browned and the fruit mixture is bubbling. Serve warm with vanilla ice cream or whipped cream.