1. Split loaf in half horizontally. Scoop out soft centers, making top and bottom bread shells about 1/2 inch thick. Reserve centers for another use. Set bread shells, cupped side up, on a 12- by 15-inch baking sheet. Bake in a 400° oven until bread is toasted, about 8 minutes.
2. Line bread shell cavities with mozzarella, overlapping slices as needed, and draping onto the rim of the bread.
3. In a 10- to 12-inch nonstick frying pan over high heat, frequently stir meat, mushrooms, and oregano until meat is browned and crumbly, 7 to 8 minutes. Add half the marinara sauce, half the olives, 3/4 cup onions, and salt and pepper to taste.
4. Fill bread shells equally with meat mixture. Top meat filling with remaining marinara sauce, olives, and onions. Sprinkle with parmesan.
5. Bake in a 400° oven until hot throughout, 10 to 15 minutes. Cut crosswise into 2- to 3-inch sections.