Make oat mixture: Combine dry ingredients in a bowl. Whisk together wet ingredients in a measuring jug. Add the wet to the dry ingredients and stir until well combined. Place in a lightly greased 7x11 inch baking dish.
Chill: Cover the dish and chill the oatmeal in the fridge overnight (you can also bake it straight away, check the notes for instructions).
Bake: When ready to bake, preheat the oven to 350°Bake the oatmeal for around 35 minutes, or until done in the middle. Cool in the pan for 5-10 minutes, then slice to serve.
Notes
To bake the oatmeal without chilling overnight, let it stand on the counter while the oven preheats. Then bake, checking after 25-30 minutes. It will be less cake-like, but still delicious!