Before bedtime, place milk and/or water, oats and optional add-ins in a 2-quart casserole or bowl that fits in your slow cooker.
Fill your slow cooker with a couple inches of water and place the casserole in the slow cooker. You want the dish to float. Carefully, add a little more water if the dish sits on the bottom of the cooker. You may also place the casserole on a rack or a ring made of foil to keep the casserole dish off the bottom of the cooker, still adding an inch or two of water to the cooker. This will cause the oats to cook more slowly, which you want.
Cook on low for 6 hours ideally, but up to 8 hours. My cooker will change to “warm” setting after the allotted cooking time is up, but I have cooked for just over 8 hours as an experiment and the oats are still delicious, perhaps a bit creamier.
Portion the oatmeal into bowls and top as desired.