Brush the butter evenly across the bottom of the stainless-steel inner pot using a pastry brush.
Add water, oats, and salt and stir to combine. Pour milk on top, but don't stir.
Close and lock lid (making sure that valve is set to seal), cook on manual for 7 minutes. When cooking time is done, let it be in warm mode for 10 minutes.
Open the lid and stir in brown sugar, chopped walnuts (if using), and a pinch of cinnamon. If the oatmeal seems too thick, loosen it with a splash of milk or water, stir gently, and serve immediately.
Video
Notes
Instant Pot Size – This recipe has been tested in both 3-quart and 8-quart instant pots.