Place bread halves, cut side up, on a baking sheet. Pull out some of soft bread centers to make loaves slightly concave. Spread each half with 1/2 cup of pasta sauce. Arrange mushrooms over sauce; sprinkle with pepperoni. Top with mozzarella; sprinkle with oregano, salt and pepper. Spoon remaining pasta sauce over tops.
Bake in preheated oven until lightly browned, about 15 minutes. Sprinkle with Parmesan cheese. Cut on angle into slices. Serve hot.