In a large mixing bowl, stir together the old-fashioned oats, natural peanut butter, ground flaxseed, mini chocolate chips, honey, and vanilla extract. Mix until evenly combined, creating a cohesive mixture.

Cover the bowl with plastic wrap or a lid and place it in the refrigerator. Allow it to chill for 30 minutes, which helps the mixture firm up for easier rolling.

After chilling, remove the mixture from the fridge. With clean hands, roll the mixture into small balls, about 1 inch in size.

Place the protein balls in an airtight container. Store in the refrigerator for up to a week.