Heat oven to 350ºF. Spray 13 x 9-inch metal baking pan with nonstick cooking spray. For topping, combine oats, brown sugar and cinnamon. Cut in butter until mixture is crumbly; set aside. For cake, combine cake mix, oats, yogurt, eggs, oil and water in large bowl. Blend on low speed of electric mixer until moistened; mix at medium speed for 2 minutes. Stir in apples. Pour into pan. Sprinkle topping evenly over batter. Bake 40 to 45 minutes or until wooden pick inserted in center comes out clean. Serve warm or at room temperature with whipped cream, if desired.