Yes, those are O.E.M. parts but the prices at that place are rather high. You'll do better at some place like Boats.net or Partzilla .....
http://www.boats.net/parts/search/Yamaha/Motorcycle/1980/XS650SG/HANDLEBAR CABLE/parts.html
http://www.boats.net/parts/search/Yamaha/Motorcycle/1980/XS650SG/HANDLEBAR CABLE/parts.html