straighten has 8 distinct synonyms spread across 4 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. Its one recorded opposite is bend.
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.
make straight
get up from a sitting or slouching position
“The students straightened when the teacher entered”
make straight or straighter
“Straighten this post”
Synonyms, antonyms and the broader and narrower relations for straighten come from Princeton University’s WordNet. Full sources and licences.