In a large bowl, combine beef, crumbs, egg, onion, salt and oregano. Stir in ¾ cup tomato sauce; set aside.
Cut two pieces of waxed paper 15 in. long.
Place meat mixture in center of one piece; cover with second piece.
With a rolling pin, press meat into a 13-in. x 9-in. rectangle.
Remove top sheet of waxed paper.
Sprinkle cheese and mushrooms over meat; top with remaining tomato sauce.
Roll up meat, jelly-roll fashion, starting at narrow end and using waxed paper to help. Seal ends securely.
Place with seam side down in a nonstick 9-¼-in. x 5-½-in. x 2-¾-in. loaf pan.
Bake at 375° for 30 minutes.
Drain off any fat; return to the oven for 30 minutes.