Preheat oven to 350°F and lightly grease a 2-quart casserole dish.
In a bowl, combine apples, cranberries, sugar, and flour. Stir to coat evenly, then scoop into baking dish, scattering any extra sugar from the bowl over the top.
In another bowl, combine oatmeal, flour, brown sugar, walnuts, cinnamon, nutmeg, and salt. Stir to evenly distribute, breaking up any chunks of brown sugar. Add the melted butter and stir until no dry bits are left and topping is forming clumps. Scatter clumps evenly over the fruit.
Bake for 45 minutes, until topping is browned and crisp and fruit juices are bubbly. Let cool at least 10 minutes before serving.