Preheat your oven to 375°F (190°C).
In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, egg, minced garlic, oregano, salt, and pepper. Mix until just combined.
Shape the mixture into meatballs, about 1-1.5 inches in diameter, and place them on a baking sheet.
Bake the meatballs in the preheated oven for 20-25 minutes, or until cooked through.
While the meatballs are baking, prepare the garlic butter by melting the unsalted butter and mixing it with minced garlic.
Slice the hoagie rolls in half and brush the garlic butter generously on the insides.
Once the meatballs are done, warm the marinara sauce in a saucepan over medium heat.
Add the baked meatballs to the sauce and stir gently to coat them evenly.
Stuff the meatballs into the buttered hoagie rolls, making sure to add a generous amount of marinara sauce.
Top each sub with shredded mozzarella cheese.
Place the prepared subs on a baking sheet and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Remove from the oven and sprinkle chopped parsley on top for garnish.
Serve warm and enjoy your indulgent meal.