Preheat the oven to 350 degrees F.
Spray 9" baking dish with cooking spray. You can also use an 8x8" baking dish.
In a large bowl, combine oats, baking powder, cinnamon, nutmeg, and salt and stir well to combine.
Add the eggnog, egg, maple syrup, and vanilla and stir well to combine.
Stir in the melted butter.
Transfer oatmeal to prepared baking dish.
Bake in the preheated oven for 30-35 minutes, until the edge is slightly golden and the middle is no longer jiggly/wet looking.
Serve topped with yogurt, additional eggnog, milk, or nuts!