The crane crushed the man at about 9:30 a.m. at Berth 37 of the port, Cal-OSHA spokesman Peter Melton said.
The man, whose name has not been released, suffered a crushed chest and head injuries, Melton said. The worker was transported to a hospital where he was pronounced dead, he said.
The worker, who was in his 50s, was a crane mechanic, Melton said. It is unknown what he was doing at the time of the incident, he said.
As of 3:30 p.m., Cal-OSHA investigators were at the scene trying to determine a cause, but Melton said they believe it was an accident.
Berth 37 is one of seven terminals at the port, and has been shut down for the day, Port of Oakland spokesman Robert Bernardo said. The remaining six terminals remained open, he said.