The shooting happened Thursday evening on the edge of the plaza outside Oakland's City Hall.
A preliminary investigation suggests the incident happened during a fight between two groups of men at or near the camp.
It's not clear yet whether those involved were part of the demonstration, but there were reports that some protest participants tried to break up the fight.
The incident adds even more strain to relations between officials and anti-Wall Street protesters.
Camp organizers said the shooting was not related to their activities, but city and business leaders said the murder is proof that the camp breeds crime and has drained law enforcement resources.
No suspects have been identified. People participating in the protest who may have taken photographs or video that captured the shooting were asked to contact authorities.
Meantime, officials also are investigating an apparent suicide of a military veteran at an Occupy encampment in Burlington, Vt. Police said preliminary investigations show that the 35-year-old fatally shot himself in the head Thursday.