offside has 1 distinct synonym spread across 1 of its 3 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 onside.
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.
(sport) the mistake of occupying an illegal position on the playing field (in football, soccer, ice hockey, field hockey, etc.)
illegally beyond a prescribed line or area or ahead of the ball or puck
“the touchdown was nullified because the left tackle was offside”
1 of the 3 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 offside.
Synonyms, antonyms and the broader and narrower relations for offside come from Princeton University’s WordNet. Full sources and licences.