In a large mixing bowl, combine the rolled oats, almond milk, cocoa powder, maple syrup, vanilla extract, and salt. Stir until well combined.
If using, fold in the Greek yogurt for extra creaminess.
Add the chocolate chips and walnuts, mixing gently to distribute evenly.
Divide the mixture into four jars or airtight containers.
Cover 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.
Serve chilled, topped with additional chocolate chips, walnuts, or fresh fruit if desired.