In a mason jar or airtight container, combine the old-fashioned rolled oats, milk, Greek yogurt, chia seeds (if using), honey or maple syrup (if using), vanilla extract, and a pinch of salt. Stir well to combine all the ingredients.

Cover the mason jar or container with a lid and shake it vigorously until everything is well mixed. Alternatively, you can stir the ingredients together with a mixing spoon or spatula.

Place the sealed mason jar or container in the refrigerator and let it chill overnight, or for at least 4 hours, to allow the oats to soften and absorb the liquid.

The next morning (or when ready to eat), give the overnight oats a good stir. If the mixture is too thick for your liking, you can add a splash of additional milk to reach your desired consistency.

Top the overnight oats with your favorite toppings, such as fresh fruit, nuts, seeds, nut butter, shredded coconut, or anything else you like.

Enjoy your delicious and nutritious overnight oats straight from the mason jar or transfer them to a bowl if preferred.