To prepare the filling: In a medium bowl, combine the sugar, flour, and lemon peel. Mix well. Add the lemon juice, apples, and cranberries; stir to mix. Spoon into a 6-cup baking dish.
To prepare the topping: In a small bowl, combine the oats, brown sugar, whole-wheat flour, and cinnamon. Add the melted margarine; stir to mix.
Sprinkle the topping over the filling. Bake for 40 to 50 minutes, or until the filling is bubbly and the top is brown. Serve warm or at room temperature.