straighten out has 23 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.
change for the better
“The lazy student promised to reform”
make straight
make free from confusion or ambiguity; make clear
“Could you clarify these remarks?”
Synonyms, antonyms and the broader and narrower relations for straighten out come from Princeton University’s WordNet. Full sources and licences.