In a large bowl, mix together old-fashioned oats, chia seeds, milk, Greek yogurt, maple syrup (or honey), ground cinnamon, and vanilla extract until well combined.
In a small bowl, mix together the ingredients for the cinnamon swirl until a paste forms.
Fill two 8 oz. jars halfway with oat mixture. Add half of the cinnamon swirl mixture to each jar. Top with remaining oat mixture and lightly swirl with a knife or spoon, so cinnamon swirl runs throughout oats.
Refrigerate overnight, or for at least 8 hours.
When ready to enjoy, top with chopped pecans, vanilla icing, and/or additional cinnamon.