Preheat the oven to 375°F and grease an 8x8-inch baking dish.

In a large bowl, toss the rhubarb with the sugar, cornstarch, and orange juice and zest. Spread evenly in the prepared baking dish.

Make the topping: In a medium bowl, combine the flour, brown sugar, oats, walnuts, cinnamon, and salt. Use your hands to work in the butter until the mixture is crumbly and holds together when pinched.

Sprinkle the topping over the rhubarb mixture and bake for 25 to 35 minutes, or until the topping is golden brown and the rhubarb is soft and bubbling.

Remove from the oven and let cool for 5 minutes. Serve with vanilla ice cream.