To make the meatballs, in a large bowl, combine the ground beef, egg, breadcrumbs, and oregano, season with salt and pepper, and mix well. Shape into 4cm balls.
In a large frying pan, heat 30ml of olive oil over medium heat and cook the meatballs until well-browned and thoroughly cooked, about 10 minutes.
To make the guacamole, in a medium bowl, mash the avocado with the remaining olive oil, parsley, chopped pepper, and mustard. Serve with the meatballs.