Cut the top third off the loaf of bread; carefully hollow out top and bottom, leaving a 1-in. shell. (Discard removed bread or save for another use.) Set loaf aside.
In a large skillet, cook the sausage, mushrooms, onion and green pepper over medium heat until meat is no longer pink; drain. In a large bowl, beat the cream cheese, Parmesan cheese, garlic powder and oregano. Stir in the sausage mixture.
Line the bottom bread shell with mozzarella slices; top with sausage mixture. Replace bread top; wrap in foil. Place on a baking sheet. Bake at 350° for 15-20 minutes or until heated through. Cool for 5 minutes before slicing.