on has 1 distinct synonym spread across 1 of its 5 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. Its one recorded opposite is off.
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.
(of events) planned or scheduled
“the picnic is on, rain or shine”
2 of the 5 recorded senses carry no synonym, antonym or related term, so they are not shown above. Every sense, with its definition and examples, is on the full entry for on.
Synonyms, antonyms and the broader and narrower relations for on come from Princeton University’s WordNet. Full sources and licences.