Preheat oven to 350°. In a large bowl, combine blueberries, rhubarb, sugar and flour. Transfer to a greased 13x9-in. baking dish., For topping, in a large bowl, combine oats, brown sugar, flour, nutmeg and cinnamon; cut in butter until crumbly. Sprinkle over fruit mixture., Bake 45-55 minutes or until the fruit is bubbly and topping is golden brown. Let cool 10 minutes. Serve warm; dollop with whipped cream if desired.