ostentation has 9 distinct synonyms spread across 2 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
lack of elegance as a consequence of being pompous and puffed up with vanity
pretentious or showy or vulgar display
Synonyms, antonyms and the broader and narrower relations for ostentation come from Princeton University’s WordNet. Full sources and licences.