Heat oven to 350°F. Generously grease and flour 13x9-inch pan. Lightly spoon flour into measuring cup; level off. In small bowl, combine flour, oats and brown sugar; mix well. With fork or pastry blender, cut in 1/2 cup margarine until mixture resembles coarse crumbs. Set aside.
In large bowl, combine all cake ingredients except apples; beat at low speed until moistened. Beat 2 minutes at high speed. Pour batter into greased and floured pan. Arrange apple slices over top of batter. Sprinkle with topping.
Bake at 350°F. for 38 to 43 minutes or until deep golden brown. Cool 5 minutes before serving.
In small saucepan, combine sauce ingredients. Cook over medium heat until warm, stirring constantly. Serve warm sauce over cake.