Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking dish.
Mix oatmeal, baking powder, and sugar together in a large bowl. Beat milk and egg together in a separate bowl; add to oatmeal mixture and stir to incorporate. Stir cranberries, coconut, and nuts into the oatmeal mixture; spread into the prepared baking dish.
Bake in preheated oven until beginning to brown along the edges, 50 to 60 minutes.