In a saucepan, combine sugar and cornstarch. Gradually stir in water and orange juice. Cook and stir until thickened and clear. Add rhubarb and bring to a boil. Remove from the heat. Pour into a 2-qt. baking dish.
For dumplings, combine flours, oats, baking powder and salt in a bowl. Combine milk and oil; add to flour mixture, stirring lightly until blended. Spoon over rhubarb. Combine topping ingredients; sprinkle over dumplings. Bake at 425° for 40 minutes.