Spray slow cooker with non-stick cooking spray to avoid oats from sticking when cooking.
In a large bowl add ingredients, except walnuts and stir together to mix.
Add ingredients to slow cooker and cook on high for 2 hours, then reduce to the low setting for 2 hours. You can also choose to cook this overnight by putting on low setting for 7 hours.
Remove from crockpot and top with walnuts. You may also choose to drizzle maple syrup or honey on top to give it extra flavor.