In a mason jar or container with a lid, combine oats, milk, and Greek yogurt. Give it a good stir to make sure everything is well mixed.
Add maple syrup, cinnamon, nutmeg, and salt. Stir again.
Fold in the diced apple. I like to reserve a few pieces for topping in the morning.
Seal the container and refrigerate overnight (or at least 6 hours).
In the morning, give everything a good stir. If it's too thick, add a splash of milk.
Top with your favorite additions and enjoy!