Mix beef, eggs, crumbs, milk, parmesan cheese, onion, salt and oregano. Shape into a loaf and place in a greased 9x5x3 loaf pan. Bake at 350 for 1 1/4 hours or until no pink remains; drain. Spoon pizza sauce over loaf and top with cheese slices. Return to the oven for 10 minutes or until cheese is melted. Meanwhile, if desired saute green pepper and mushrooms in butter; arrange on top of meatloaf.