Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper.
In a large bowl, combine ground beef, breadcrumbs, Parmesan, egg, garlic, oregano, salt, pepper, and parsley. Mix until just combined.
Shape the mixture into meatballs about 1 to 1 1/2 inches in diameter and place them on the prepared baking sheet.
Bake for 18-20 minutes or until browned and cooked through.
While the meatballs are baking, heat marinara sauce in a large skillet over medium heat.
Add cooked meatballs to the sauce and simmer for 5 minutes.
Slice sub rolls without cutting all the way through. Place 3–4 meatballs in each roll, spoon sauce over the top, and sprinkle with mozzarella.
Place subs on a baking sheet and broil for 2–3 minutes or until cheese is melted and bubbly. Serve warm.