Combine the first 3 ingredients in a bowl, and toss well. Spoon into an 8-inch square baking dish coated with cooking spray.
Place oats, flour, and sugar in food processor, and pulse 2 to 3 times. Add chilled margarine, and process until mixture resembles coarse meal; sprinkle over rhubarb mixture. Bake at 375° for 40 minutes or until rhubarb is tender. Serve with frozen yogurt.