In a large bowl, combine the ground beef, ground pork, breadcrumbs, Parmesan cheese, parsley, garlic, egg, salt, black pepper, oregano, and red pepper flakes. Mix until well combined, but do not overmix.
Shape the mixture into meatballs, about 1 to 1.5 inches in diameter.
In a large skillet, heat the olive oil over medium heat. Add the meatballs in batches, being careful not to overcrowd the pan. Cook for about 5-7 minutes, turning occasionally until browned on all sides.
Once browned, add the marinara sauce to the skillet, covering the meatballs. Reduce the heat to low and simmer for 20 minutes, allowing the flavors to meld and the meatballs to cook through.
Serve the meatballs hot, garnished with extra parsley and Parmesan cheese if desired.