Yes, you can totally do this to save on time, but I personally like to prep and bake it all at once because the oats do tend to soak up quite a bit of the milk overnight (kind of like overnight oats). If you don't have a choice and need to prep the oatmeal bake ahead of time, that's fine. It might just seem a little bit drier than normal after baking. If this happens, just serve with a splash of milk. Another prep-ahead option is to combine all the dry ingredients in a bowl and combine the wet ingredients in a separate bowl the night before. The morning of, simply mix the two together, pour into your baking dish and bake!