In a glass jar or container, combine the rolled oats, 1 cup of the Greek yogurt, chia seeds, cinnamon, vanilla extract, and water.
Stir the mixture thoroughly until well combined and the chia seeds are evenly distributed.
Cover the container and refrigerate for at least 4 hours, or preferably overnight, to allow the oats to soften into a velvety texture.
While the oats soak, hard-boil the egg by placing it in a small pot of water, bringing it to a boil, then simmering for 9 minutes before cooling in an ice bath.
In the morning, dice the half apple into small pieces.
Top the chilled oats with the remaining 0.25 cup of Greek yogurt and the fresh diced apples.
Serve the overnight oats alongside the peeled hard-boiled egg for a complete, protein-packed breakfast.