In a mason jar or small container with a lid, add oats, milk, Greek yogurt, chia seeds, maple syrup, and vanilla extract. Stir well until completely combined.
Cover and refrigerate overnight, or for at least 6 hours, to let the oats soften and the mixture thicken.
In the morning, stir the oats again.
Top with strawberries, blueberries, raspberries, and blackberries.
Garnish with a spoonful of Greek yogurt, a drizzle of honey, and a sprinkle of chia seeds.