Add oil to a large skillet on medium-high heat. Once the oil starts to shimmer, add in meatballs and sauté for 3-5 minutes. Turn the meatballs every so often to brown the outside.
Preheat the broiler. Add 4 meatballs to each roll. Top with marinara sauce and ¼ cup of mozzarella. Set the baking sheet under the broiler and allow to cook until the cheese melts.