The Collaborative International Dictionary of English v.0.48:

Ostentatious \Os`ten*ta"tious\, a.
   Fond of, or evincing, ostentation; unduly conspicuous;
   pretentious; boastful.
   [1913 Webster]

         Far from being ostentatious of the good you do.
                                                  --Dryden.
   [1913 Webster]

         The ostentatious professions of many years. --Macaulay.
   [1913 Webster] -- Os`ten*ta"tious*ly, adv. --
   Os`ten*ta"tious*ness, n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ostentatious
    adj 1: intended to attract notice and impress others; "an
           ostentatious sable coat" [syn: ostentatious,
           pretentious] [ant: unostentatious, unpretending,
           unpretentious]
    2: (of a display) tawdry or vulgar [syn: ostentatious,
       pretentious]

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "ostentatious":
   Gongoresque, Johnsonian, affected, ambitious, arabesque, baroque,
   bedizened, big-sounding, boastful, braggart, busy, chichi, classy,
   convoluted, declamatory, elaborate, elegant, elevated, euphuistic,
   fancy, fine, flamboyant, flaming, flash, flashy, flaunting, florid,
   flossy, flowery, frilly, fulsome, fussy, garish, gaudy,
   grandiloquent, grandiose, grandisonant, high-flowing, high-flown,
   high-flying, high-sounding, high-toned, high-wrought, highfalutin,
   highfaluting, inkhorn, labored, labyrinthine, lexiphanic, lofty,
   lurid, luxuriant, luxurious, magniloquent, meretricious, moresque,
   ornate, orotund, overdone, overelaborate, overelegant,
   overinvolved, overlabored, overworked, overwrought, peacockish,
   peacocky, pedantic, picturesque, pompous, pretentious,
   pretty-pretty, rhetorical, rich, rococo, sensational,
   sensationalistic, sententious, showy, sonorous, splashy, stilted,
   swank, tall, theatrical, tony, tortuous, vain, vainglorious,
   vaunting