openness has 3 distinct synonyms spread across 2 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 closeness.
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.
characterized by an attitude of ready accessibility (especially about one's actions or purposes); without concealment; not secretive
willingness or readiness to receive (especially impressions or ideas)
“he was testing the government's receptiveness to reform”
Synonyms, antonyms and the broader and narrower relations for openness come from Princeton University’s WordNet. Full sources and licences.