Heat the oil in a heavy large frying pan over medium-high heat. Add the meatballs and saute until browned on all sides, about 5 minutes. Turn off heat. Transfer the meatballs to a plate. Pour off any excess oil. Add the marinara sauce, about 3 cups. Return all the meatballs to the pan. Turn the heat to medium-low and simmer until the sauce thickens slightly and the flavors blend, 15 to 20 minutes. Season the sauce, to taste, with salt and pepper.
Transfer the meatball mixture to a serving bowl. Serve with toothpicks.