WordNet records no synonym and no antonym for co-opt. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
choose or elect as a fellow member or colleague
“The church members co-opted individuals from similar backgrounds to replenish the congregation”
neutralize or win over through assimilation into an established group
“We co-opted the independent minority tribes by pulling them into the Northern Alliance”
Synonyms, antonyms and the broader and narrower relations for co-opt come from Princeton University’s WordNet. Full sources and licences.
appoint summarily or commandeer
“The army tried to co-opt peasants into civil defence groups”
take or assume for one's own use
“He co-opted the criticism and embraced it”