Preheat the oven to 400 degrees. In a mixing bowl combine the ground turkey, breadcrumbs, egg, garlic powder, oregano, salt, and black pepper. Mix until evenly blended then shape into 16 small meatballs.
Arrange the meatballs on a baking sheet and bake for 15 to 18 minutes until they reach an internal temperature of 165 degrees.
While the meatballs cook, warm the marinara sauce in a skillet over medium heat until it begins to bubble.
Split each hoagie roll and place four meatballs inside. Spoon warm marinara over the meatballs.
Sprinkle mozzarella cheese on top of each sub and place them under the broiler for 2 to 3 minutes until the cheese melts and starts to brown.