In a large bowl, toss the rhubarb, sugar and tapioca; let stand for 15 minutes, stirring occasionally. Pour into a greased 13x9-in. baking pan. Top with the oranges. , In a large bowl, combine the brown sugar, oats, flour and salt. Cut in butter until mixture resembles coarse crumbs; sprinkle evenly over oranges. , Bake at 350° for 40 minutes or until top is golden brown. Serve with ice cream if desired.