offstage has 2 distinct synonyms spread across 2 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 onstage.
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 in the area of a stage not visible to the audience
“offstage noises”
behind the scenes; not on stage
“the actors were waiting offstage”
1 of the 5 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for offstage.
Synonyms, antonyms and the broader and narrower relations for offstage come from Princeton University’s WordNet. Full sources and licences.