Osanlu has led the union for two years and has reportedly helped to reorganize and reactivate the union -- which is not legally recognized by the Iranian government.
But he has paid a high price for his involvement in the union.
Because of his trade-union activities, Osanlu has been subjected to pressure by the Iranian authorities, who have detained, summoned him to court, and jailed him several times over the last 18 months.
Osanlu was first detained in December 2005 for organizing a protest action by bus drivers complaining of low pay. He then spent about seven months in Tehran's notorious Evin prison with almost no contact to the outside world.
In January 2006, bus drivers and union activists planned a strike to call for his release and also press for other demands, including a pay increase. But on the eve of the planned strike the Iranian government preemptively detained hundreds of bus drivers and several members of the union's executive committee.
In July 2006, dozens of rights activists in Iran and elsewhere went on a three-day hunger strike to call for his unconditional release as well as the release of other Iranian citizens who are held because of their peaceful activities or opinions not tolerated by Iranian officials.
Activists said they decided to highlight Osanlu's case as a symbol of Iran's workers' movement, which is under increasing state pressure.
Several human rights groups, including Amnesty International, also called for his release.
Many Iranian rights activists and intellectuals issued a statement in his support.
Osanlu was finally freed in the summer of 2006 after posting a bail of about $165,000.
However, state pressure on him continued.
Osanlu was detained by officials several times, including in November 2006, when he was taken away from a Tehran street by plainclothes security agents who were reportedly from Iran's Intelligence Ministry.
He was put on trial in February 2007.
Charges brought against Osanlu included "engaging in publicity against Iran's government" and acting against domestic security.
He however told Radio Farda that authorities have turned a "simple trade union and workers' case" into a "security matter."
His July 10 abduction set off alarms among human rights groups and organizations defending union rights.
Two days later officials told his family that he is jailed in Tehran’s notorious Evin prison.
The reason for his rearrest and jailing is not clear.
But some observers believe it could be because of his recent trip to Europe. During his visit, Osanlu described the government's pressure and harassment against him and other workers.
Osanlu was quoted, however, as saying that he will not be intimidated.