Lightly grease the rice cooker pot to prevent sticking.
Add rolled oats, water or milk, banana slices, mixed berries, chopped nuts, honey or maple syrup (if using), cinnamon (if using), and a pinch of salt into the rice cooker.
Close the lid and set the rice cooker to the 'Cook' setting. Let it cook until it automatically switches to 'Warm'.
After it switches to 'Warm', let it sit for 5-10 minutes to thicken.
Open the lid, stir gently. If oatmeal is too thick, add a splash of water or milk.
Scoop oatmeal into bowls, top with extra banana slices, berries, nuts, and a drizzle of honey or maple syrup.