Preheat the oven to 350 degrees F.
Combine the rhubarb, cranberries, sugar, and cornstarch in a large cast iron skillet (or baking dish). Stir everything well to combine.
Prepare the topping by combining the oats, flour, brown sugar, cinnamon, salt, walnuts, and softened butter in a large bowl. Mix well.
Spread the topping over the filling in the skillet.
Bake in the oven at 350 degrees F for about 40minutes, or until topping is golden and crisp and the filling is soft. Let cool slightly, then enjoy.