In a mason jar or airtight container, combine the rolled oats, milk, Greek yogurt, maple syrup, vanilla extract, ground cinnamon, chia seeds (if using), and a pinch of salt.
Stir everything together thoroughly with a spoon until the oats are fully coated and the cinnamon is evenly distributed throughout the mixture. Make sure no dry oats remain at the bottom.
Seal the container tightly with a lid. If using a mason jar, give it a good shake to ensure everything is well combined.
Refrigerate for at least 4 hours, or preferably overnight (8-12 hours), allowing the oats to absorb the liquid and soften to a creamy, pudding-like consistency.
In the morning, give the oats a quick stir. If they're too thick, add a splash of milk to reach your desired consistency.
Top with your favorite additions such as fresh berries, sliced banana, chopped nuts, a drizzle of maple syrup, or a spoonful of nut butter. Enjoy cold or microwave for 45-60 seconds if you prefer them warm.