Colombia's main oil workers' union, the USO, will hold an indefinite strike, its president told Reuters on Wednesday, in protest at widespread job cuts in the sector after a plunge in crude oil prices, but no start date has yet been set.   A date for the stoppage would be fixed by March 26, said Edwin Castano moments after a meeting with members to vote on strike action. The union was still willing to discuss members' grievances with the government in the meantime, he said.   More than half Colombia's oil is produced by state-controlled Ecopetrol while Toronto-listed Pacific Rubiales is the biggest private player. Around 2,400 USO members work at Ecopetrol and some are contractors at Pacific.   "The 26th of March is the deadline we have for setting when the strike will begin. Meanwhile, we're waiting to open dialogue with the government and with Ecopetrol," Castano said.   Labor Minister Luis Eduardo Garzon said the government was willing to hear the union's demands and explore solutions, during a meeting with Castano on Tuesday, but members voted to strike nonetheless.   An Ecopetrol source told Reuters the company is preparing a contingency plan to minimize disruption in the event of a stoppage and said strikes at Ecopetrol are prohibited by law because the state-owned firm provides a service to the public.   A strike would deal a further blow to the Andean nation's million-barrel-per-day industry already reeling from the roughly halving of global crude prices since last June. It would also cut badly-needed royalty and tax revenue the government relies on and hit oil companies' earnings.   Around 10,000 workers have been laid off in the past few months and dismissals could reach 25,000 this year, Castano said, as companies rethink their investment and production plans with less cash to invest.   Crude is Colombia's largest export and source of foreign exchange. The Andean country is the fourth-largest oil producer in Latin America.   The union also objects to the idea that the government could at some point sell more shares in state-run Ecopetrol and to a plan under way to privatize publicly-owned Isagen, in a bid to raise funds for highway infrastructure.