Generously spray your slow cooker walls with non-stick spray or line with a slow cooker liner.
Place almond milk, oats, vanilla, cinnamon, and diced apples in the slow cooker and stir to combine. Cook on low for 6-8 hours, until the oats are soft and cooked through.
Serve & store:
Serve sprinkled with brown sugar and additional chopped apple.
Leftover oatmeal can be stored in an airtight container in the fridge for up to 5 days or in the freezer for up to 3 months.