HEAT oven to 350°F. Toss apples with 1/4 cup of the brown sugar and 1 tsp. of the cinnamon. Spoon into 8- or 9-inch square baking dish.
COMBINE oats, the remaining 1/4 cup brown sugar and 1 tsp. cinnamon in medium bowl. Cut in margarine with pastry blender or two knives, until mixture resembles coarse crumbs. Add biscuit crumbs; mix well. Spread crumb mixture evenly over apples.
BAKE 30 to 35 min. or until apples are tender. Serve warm or cooled. Top with whipped topping.