In a small bowl that fits inside you Instant Pot, mix together the oats, milk, 2 cups of water, and cinnamon stick.
Pour the remaining 1 1/2 cups of water into the inner pot, and place the trivet inside.
Place the bowl of oat/milk mixture on the trivet, and secure the lid.
Select Pressure Cook on High and set the time to 5 minutes.
Once the cooking program has finish, allow the pressure to release naturally for 10 minutes, then quick release any remaining pressure.
Unlock and remove the lid. Remove the bowl, and mix the oats thoroughly.
Stir in the sugar, 1 chopped apple, ground cinnamon, and walnuts.
Top with the remaining 1 chopped apple and serve warm.