In bowl, combine flour, oats, brown sugar, cinnamon and nutmeg; stir in butter. Press half of the mixture over bottom of greased 9-inch square cake pan.
Filling: In bowl, toss together chopped rhubarb, strawberries, apples and blueberries; sprinkle with sugar and add orange juice and zest.Arrange over base. Sprinkle with remaining flour mixture. Bake in 350°F (180°C) oven for 45 to 50 minutes or until fruit is tender and topping is golden. Serve warm.