Osanloo, the president of the Syndicate of Workers of the Tehran Bus Company, was released on December 19.
Before his arrest in November he had spent more than seven months in Tehran's Evin prison on unknown charges.
Osanloo was arrested in December 2005 after organizing protests against the working conditions of bus drivers.
Rights groups and international organizations defending the rights of workers had expressed concern over his fate.
(with material from ILNA)