Locate an oven-safe, not-too-shallow baking dish (such as Corningware or Pyrex) that will fit in your slow cooker.
In the baking dish, place oats, diced apples, apple juice, milk, maple syrup, vanilla, cinnamon, nutmeg, and salt; stir to combine.
Carefully place baking dish down in slow cooker and slowly pour water into the bottom of the slow cooker until its level is about 1 inch below the top of the baking dish.
Cover and cook on LOW for 8 hours.
Stir before serving.
