Combine everything in a jar. Add the oats, milk, Greek yogurt, chia seeds, maple syrup, vanilla, and salt to a mason jar or any container with a lid. Stir really well so there are no dry pockets of oats hiding at the bottom.
Give it a second stir. After 2-3 hours in the fridge (or right before bed), give the oats another quick stir. This keeps the chia seeds from sinking to the bottom and evens out the texture.
Refrigerate overnight. Cover and refrigerate for at least 4 hours — overnight (7-8 hours) is ideal. The oats soften, the chia plumps up, and everything melds together.
Stir and adjust in the morning. Give it a good stir. If it's too thick for your liking, add a splash of milk until it reaches your perfect consistency.
Add toppings and enjoy. Top with fresh fruit, a drizzle of nut butter, or a sprinkle of nuts. Eat it cold straight from the fridge — or warm it up (see FAQs below).