onstage has no recorded synonym. 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 offstage.
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.
situated or taking place on the area of a stage visible to the audience
on the stage
“it was time for her to go onstage”
Synonyms, antonyms and the broader and narrower relations for onstage come from Princeton University’s WordNet. Full sources and licences.