In a large bowl, combine the ground beef, breadcrumbs, Parmesan cheese, parsley, egg, garlic powder, onion powder, salt, and pepper.
Gently fold in the cubed mozzarella cheese.
Form the mixture into meatballs, about 1.5 inches in diameter.
Place the meatballs in the slow cooker, making sure they are not overcrowded.
Pour the marinara sauce over the meatballs, ensuring they are well-coated.
Sprinkle the dried oregano over the sauce.
Cover the slow cooker and cook on low for 6-8 hours, or on high for 3-4 hours.
Once cooked, gently remove the meatballs from the slow cooker using a slotted spoon.
Serve the meatballs with the marinara sauce, either over pasta, in a sub, or on their own.