Preheat oven to 180 C / Gas mark 4.
To make apple filling: In a large saucepan combine water, cinnamon, sugar and cornflour. Mix well, then add apples to pan. Simmer over medium heat, stirring occasionally, until apples are tender. Pour apple filling into a 23x33cm (9x13 in) baking dish.
To make topping: In a small bowl, combine oats, melted butter and brown sugar. Mix well and sprinkle over apple filling.
Bake in preheated oven for 20 minutes or until topping is golden brown.