In a large mixing bowl, combine the rolled oats, almond milk, diced apple, ground cinnamon, maple syrup, vanilla extract, and salt. Stir well until all ingredients are evenly mixed.
If using, fold in the chopped walnuts and raisins for added texture and flavor.
Divide the mixture evenly into four jars or containers with lids.
Cover the jars and refrigerate overnight, or for at least 4 hours, to allow the oats to soak and soften.
In the morning, give the oats a good stir. You can add a splash of milk if you prefer a thinner consistency. Enjoy cold or warm them in the microwave for about 30 seconds if desired.