In a large mixing bowl, combine the rolled oats, peanut butter, honey, chocolate protein powder, mini chocolate chips, peppermint extract, and salt. If using walnuts, add them in as well.
Mix all the ingredients together until well combined. You may need to use your hands to fully incorporate everything.
Once the mixture is fully combined, refrigerate for about 30 minutes to help it firm up.
After chilling, scoop out tablespoon-sized portions of the mixture and roll them into balls.
Place the protein balls on a baking sheet lined with parchment paper and refrigerate for another 30 minutes to set.
Store the protein balls in an airtight container in the refrigerator for up to one week.