organ has 6 distinct synonyms spread across 3 of its 6 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.
a fully differentiated structural and functional unit in an animal that is specialized for some particular function
a government agency or instrument devoted to the performance of some specific function
“The Census Bureau is an organ of the Commerce Department”
(music) an electronic simulation of a pipe organ
a periodical that is published by a special interest group
“the organ of the communist party”
wind instrument whose sound is produced by means of pipes arranged in sets supplied with air from a bellows and controlled from a large complex musical keyboard
a free-reeded instrument with a piano keyboard in which air is forced through the reeds by bellows
Synonyms, antonyms and the broader and narrower relations for organ come from Princeton University’s WordNet. Full sources and licences.