Prepare the base: In a mixing bowl, combine the rolled oats, milk, Greek yogurt, sweetener, vanilla extract, and cinnamon. Stir well until fully combined.
Layer the parfait: Divide the oat mixture evenly into two jars or containers. Layer in your choice of fresh fruit (berries, banana slices, or diced apples). Add a sprinkle of chia seeds or flaxseeds, if desired.
Refrigerate overnight: Seal the jars with their lids and refrigerate overnight, or for at least 6 hours. The oats will absorb the liquid and become creamy.
Serve: In the morning, remove the parfaits from the fridge. Stir to mix the layers, and top with granola or chopped nuts for crunch, if desired.
Enjoy: Enjoy your delicious, nutritious overnight oats parfaits as a healthy breakfast or snack.