opinion has 15 distinct synonyms spread across 6 of its 6 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.
a message expressing a belief about something; the expression of a belief that is held with confidence but not substantiated by positive knowledge or proof
“his opinions appeared frequently on the editorial page”
a belief or sentiment shared by most people; the voice of the people
“he asked for a poll of public opinion”
the legal document stating the reasons for a judicial decision
“opinions are usually written by a single judge”
the reason for a court's judgment (as opposed to the decision itself)
a vague idea in which some confidence is placed
“his impression of her was favorable”
Synonyms, antonyms and the broader and narrower relations for opinion come from Princeton University’s WordNet. Full sources and licences.