Preheat the oven to 375°F (190°C).
In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, egg, minced garlic, chopped parsley, dried oregano, salt, and black pepper. Mix until all ingredients are well incorporated.
Shape the mixture into 16 evenly sized meatballs, about 1 1/2 inches in diameter.
Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the meatballs in batches and cook until browned on all sides, about 5 minutes. Remove the meatballs and set aside.
In the same skillet, pour in the marinara sauce and bring to a simmer. Return the meatballs to the skillet, cover, and cook for 15 minutes until the meatballs are cooked through.
While the meatballs are cooking, place the sub rolls on a baking sheet and brush the cut sides with the remaining 1 tablespoon of olive oil. Toast the rolls in the oven for 5 minutes until lightly crispy.
Remove the toasted rolls from the oven and spoon 4 meatballs with sauce into each roll.
Top each sandwich with 2 ounces of shredded mozzarella cheese.
Place the sandwiches back in the oven for 5 minutes or until the cheese is melted and bubbly.
Remove from the oven and serve hot.