TO MAKE ON THE STOVE: In a medium saucepan, combine the oats, milk, water, salt, and any desired mix-ins. Bring to a boil, then reduce the heat to a simmer. Let simmer until the oats are softened to your liking, about 5 minutes. Let stand 2 minutes, then serve with any desired toppings.
TO MAKE IN THE MICROWAVE: In a roomy microwave-safe bowl that will give you lots of room for the oatmeal to expand, combine the oats, milk, water, salt, and any desired mix-ins (to be extra safe if you aren't sure if your bowl is big enough, set it on a plate to catch any oats that boil over). Heat on high for 90 seconds, then stir and microwave until the oatmeal has bubbled up and softened, about 1 minute more. Let stand 2 minutes, then stir and enjoy hot with any desired toppings.
Notes
TO STORE: Refrigerate in an airtight container for up to 5 days.
TOREHEAT: Warm up leftover oatmeal in the microwave or on the stovetop over medium heat with a splash of water or milk to loosen it up. (You can also eat leftovers cold.)
TOFREEZE: Freeze oatmeal in a freezer-safe storage container or freezer bag for up to 3 months. Thaw overnight in the refrigerator, then reheat according to the instructions above.