Preheat oven to 350°F (180°C).
In a large bowl, stir together oats, cranberries, sugar, cinnamon, baking powder and salt.
In another bowl, stir together apple, eggs, milk, butter and vanilla. Pour apple mixture overtop of oat mixture, stirring until combined. Pour into buttered 8 inch (20 cm) baking dish. Bake for about 30 minutes or until golden brown and set.
Serve with yogurt and fresh berries if desired.
Make-ahead: Cover and refrigerate for up to 5 days. Reheat in microwave or enjoy at room temperature.