In a 12-ounce or 16-ounce glass jar, combine the rolled oats, chia seeds, cinnamon, nutmeg, and salt.
Add the milk, Greek yogurt, maple syrup, and vanilla extract to the dry mixture.
Incorporate the grated or diced apple into the liquid-oat base and stir with a spoon until the ingredients are homogenously distributed.
Seal the container with a tight-fitting lid and agitate slightly to ensure no dry pockets remain at the bottom of the vessel.
Refrigerate for a minimum of 6 hours, ideally 8 hours, to allow the starch granules to hydrate and the chia seeds to form a gel.
Stir the mixture before consumption; if the texture is too dense, incorporate 1 to 2 tablespoons of additional milk to reach the target viscosity.