In a medium bowl or storage container, stir together oats, almond milk, maple syrup, vanilla extract, cinnamon, and 1/2 of the apple, chopped. Cover and let sit in the refrigerator until the oats soak up the milk and soften, overnight or for at least 4 hours.
For this breakfast, serve 3/4 cup of the oats topped with the remaining chopped apple and walnuts.