Remove the stems from the mushrooms and reserve the caps. In a mixing bowl combine the swiss cheese, bacon, onion, olives and parmesan cheese. Blend thoroughly. Stuff the mushroom caps with the cheese mixture, making sure that the filling is slightly mounded. Place the stuffed mushrooms on a baking sheet. Bake at 350 F. about 15 minutes, then broil for about 5 minutes more or until the filling is brown. Serve warm.