Spray inside of a large slow cooker with cooking spray. Add all ingredients (except walnuts) to slow cooker. Stir, cover, and cook on LOW for 7 hours.
Serve oatmeal in bowls; add walnuts. You can also add raisins, any other fresh fruit or dried fruit, other nuts or maple syrup.
Store leftovers in refrigerator. Freeze in individual servings if desired.
To reheat single servings: Put 1-cup cooked oatmeal in microwave-proof bowl. Add 1/3 cup fat-free milk. Microwave on high for 1 minute; stir. Continue cooking for another minute, or until hot.