Add the oats, milk, honey, chia seeds (if using), and vanilla extract to a large glass jar or container with a lid.
Stir well until everything is fully combined and the oats are evenly coated.
Secure the lid and place the jar in the refrigerator overnight (or at least 6 hours).
In the morning, give the oats a quick stir. Add a splash of milk if needed to loosen the texture. Top with your favorite toppings and enjoy straight from the jar or transfer to a bowl.