Should you layer Greek yoghurt on porridge oats or mix it in with the porridge oats?
The point of overnight oats is to allow the oats time to hydrate slowly at a cool temperature, rather than cooking them in water. So it's best to have them well-mixed with the liquid they'll be absorbing. If you put them in (sufficiently thick) layers, it's likely that some of the oats will be less hydrated than others.