Preheat oven to 375°F. Lightly coat a 7-by-11-inch baking dish with cooking spray.
Using the large holes of a box grater, grate 1 apple to yield about ¾ cup packed. Set aside. Cut the remaining apple into ½-inch cubes to yield about 1 cup.
Combine 2½ cups oats, 1½ cups milk, 2 eggs, ⅓ cup brown sugar, 1 tablespoon plus 2 teaspoons orange zest, 3 tablespoons orange juice, 1 tablespoon vanilla, 1 teaspoon baking powder, ¾ teaspoon cinnamon, ¼ teaspoon nutmeg and ¼ teaspoon salt in a large bowl; stir until thoroughly combined. Fold in 2 cups cranberries and the grated apple. Transfer to the prepared baking dish and sprinkle with the chopped apple.
Bake until golden brown and set, 40 to 45 minutes. Let cool for 15 minutes. Slice and serve warm or at room temperature.
