
In a large jar or bowl, combine the quick cooking oats, chia seeds, Greek yogurt, milk, vanilla extract, and maple syrup. Stir until fully combined; the mixture will look thin at first, but the oats and chia seeds will thicken it as they chill.
If you're making a flavor variation, stir in that flavor's mix-ins now (fruit, protein powder, nut butter, or spices) so they have all night to meld with the base. Hold off on crunchy add-ins like nuts, granola, or cookie crumbles until morning so they stay crisp instead of going soft.
Cover and refrigerate for at least 2 hours, or overnight (preferred). The oats should be thick and creamy when they're ready, not soupy or watery.
When you're ready to eat, stir your overnight oats well. If they're thicker than you'd like, add a splash of milk and stir again to loosen them up.
Add the crunchy toppings you held back, then enjoy cold. If you prefer warm oats, microwave your serving for 30-60 seconds before topping and eating.