WordNet (r) 3.0 (2006):

old master
    n 1: a great European painter prior to 19th century

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "old master":
   art object, artisan, artist, artiste, brainchild, bric-a-brac,
   classic, composition, copyist, craftsman, creation, creator,
   dauber, daubster, design, grotesque, kitsch, maker, master,
   masterpiece, masterwork, mobile, museum piece, nude, pasticcio,
   pastiche, piece, piece of virtu, stabile, statue, still life,
   study, virtu, work, work of art