If making homemade sauce, follow the recipe below. If using store-bought, place the marinara sauce in a pot over medium heat, cover with a lid, and bring to a low simmer.
In a bowl, mix together the bread and milk. Let sit for 2 to 3 minutes.
Add the ground beef, oregano, parmesan cheese, salt, and black pepper to the soaked bread mixture. Mix until well combined. Form the mixture into equally-sized meatballs using your hands.
Heat a sauté pan over medium heat. Add the olive oil, then place the meatballs in batches, ensuring they are not crowded. Sear on all sides until browned.
Transfer the meatballs to the pot of marinara sauce. Simmer until the meatballs reach an internal temperature of 160°F (71°C) using a meat thermometer. Season the sauce with salt and pepper to taste, then remove from heat.
Preheat the oven broiler on low.
Drizzle the inside of each sub roll with olive oil and place them on a baking sheet. Lightly toast under the broiler until golden.
Place the meatballs on the toasted sub rolls, spoon some sauce over them, and top with shredded mozzarella.
Return the assembled subs to the broiler and heat until the cheese is melted and bubbly.
Cut the sandwich in half and serve with carrots, ranch dressing, and pink lemonade.