opposition has 8 distinct synonyms spread across 5 of its 8 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.
the action of opposing something that you disapprove or disagree with
“he encountered a general feeling of resistance from many citizens”
the relation between opposed entities
the act of hostile groups opposing each other
“the government was not ready for a confrontation with the unions”
a body of people united in opposing something
the major political party opposed to the party in office and prepared to replace it if elected
“Her Majesty's loyal opposition”
Synonyms, antonyms and the broader and narrower relations for opposition come from Princeton University’s WordNet. Full sources and licences.