In bowl combine with flour, oats, brown sugar, cinnamon and nutmeg. Stir in butter. Press half of the mixture over bottom of greased 9 in square cake pan. Sauce: In small saucepan combine sugar with cornstarch and whisk in water and orange rind until smooth. Bring to a boil and reduce heat to medium low and cook for 5 minutes more or until thickened and clear whisking constantly. Filling: In bowl toss together chopped rhubarb, strawberries, apples, and blueberries and arrange over base. Pour sauce over top and sprinkle with remaining flour mixture. Bake at 350 for 50 to 60 minutes or until fruit is tender and topping is golden. Serve warm.