Combine water, oats, apple, cinnamon, salt, and nutmeg in a multi-functional pressure cooker (such as Instant Pot(R)). Close and lock the lid. Seal vent. Select Manual function; set timer for 5 minutes. Allow 10 to 15 minutes for pressure to build.
Release pressure using the natural-release method according to manufacturer's instructions, about 10 minutes. Release remaining pressure naturally. Stir and remove pot carefully with oven mitts.
