Preheat oven to 400 degrees. In a 9-by-13-inch baking dish, combine rhubarb, sugar, and 1/4 cup flour; set aside.
In the bowl of a food processor, combine remaining 1/2 cup flour and the butter. Pulse until the butter pieces are pea-size. Add brown sugar, oats, and cinnamon. Pulse to combine. Sprinkle over rhubarb.
Bake until rhubarb is tender and topping is golden, 35 to 45 minutes. Serve warm with ice cream, if desired.