The antonyms for the phrase "obtain release of" can vary depending on the context. Generally, the antonyms involve preventing or blocking the release of something or someone, rather than actively obtaining or achieving it. Some possible antonyms include "detain," "withhold," "keep in captivity," "confine," "imprison," "retain," or "hold onto." These words all suggest hindering or obstructing the release of something or someone, rather than facilitating it. In contrast, "obtain release of" implies a goal or desire to secure freedom or liberation from captivity or restraint.