Combine dry ingredients: Add the rolled oats, chia seeds, cinnamon, nutmeg, and salt to a medium bowl or large mason jar and stir to combine.
Add wet ingredients: Pour in the almond milk, Greek yogurt, and vanilla extract, then stir well until everything is evenly mixed and the oats are fully coated.
Fold in the apples: Add the diced apple and stir gently to distribute throughout.
Divide into containers: Pour the mixture into two individual containers with lids, or keep in one large container if you prefer (each should hold about 1.5 cups).
Refrigerate overnight: Cover and refrigerate for at least 4 hours, but preferably overnight (8-12 hours) until the oats absorb the liquid and soften to a creamy, pudding-like consistency.
Serve cold: Give the oats a quick stir in the morning, add optional toppings if desired, and eat directly from the container or transfer to a bowl.
Adjust consistency: If the oats are too thick after refrigeration, stir in a splash of almond milk to reach your preferred consistency.