Mix together oats, baking powder, spices and sugar. Mix together milk, eggs and vanilla. Pour over oat mixture and let stand 5-10 minutes until most of the liquid has soaked into the oats. Pour melted butter over this and stir.
Add chopped apples to the bottom of the greased baking dish. Pour oats overtop.
Bake uncovered for 45 minutes, until golden brown.
(To make ahead, do not bake until the morning. Cover with foil and refrigerate overnight, then uncover and bake in the morning.)