In a large mixing bowl, combine the ground beef, ground pork (or more beef), breadcrumbs, Parmesan cheese, parsley, finely chopped onion, minced garlic, egg, dried oregano, salt, and pepper. Mix until just combined, being careful not to overwork the meat.
Shape the mixture into golf-ball-sized meatballs, about 1 1/2 inches in diameter. You should get around 20 meatballs.
In a large skillet, heat a generous amount of olive oil over medium heat. Once hot, add the meatballs in batches, taking care not to crowd the pan.
Cook the meatballs for about 4-5 minutes on each side or until they are golden brown and cooked through, flipping them carefully to avoid breaking.
Once browned, transfer the meatballs to a plate and repeat with the remaining meatballs, adding more olive oil as needed.
After browning, return all the meatballs to the skillet and pour the marinara sauce over them. Simmer for an additional 10-15 minutes until the meatballs are fully cooked and have absorbed some of the sauce flavors.
Serve the meatballs hot, either on their own or over spaghetti, garnished with additional parsley and Parmesan cheese if desired.
Notes
For a lighter version, use ground turkey instead of beef and pork.