obligate has 4 distinct synonyms spread across 2 of its 4 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 facultative.
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.
commit in order to fulfill an obligation
“obligate money”
restricted to a particular condition of life
“an obligate anaerobe can survive only in the absence of oxygen”
Synonyms, antonyms and the broader and narrower relations for obligate come from Princeton University’s WordNet. Full sources and licences.