Oakland Police Officer's Association President Dom Arotzarena said about three-fourths of the union's membership voted on the concessions and a strong majority of those who voted approved the concessions, although he did not release exact figures.
Arotzarena said the union's current contract does not expire until 2013, but he said it agreed to reopen it and work on a revised contract that will last until June 30, 2015, "because we realized that the city is facing a huge financial crisis."
He said Oakland police officers "are digging deep in their pockets in this time of need" and the concessions mean they will be taking home $9,000 to $15,000 less a year.
The concessions will save Oakland about $65 million over the next four years, Arotzarena said.
In addition to contributing 9 percent of their pension costs, the officers agreed to delay previously negotiated cost-of-living increases until 2014 and give up two holidays per year through 2015.
The agreement also calls for entry-level pay for new hires to be reduced by 10 percent and for the retirement age for new officers to be raised to 55 instead of the current retirement age of 50.
In return for the police union's concessions, the city, which laid off 80 officers last year, agreed not to lay off any more officers or have any officer furloughs for the next four years.
Arotzarena said, "Our concern was that our officers gave up so much that we wanted job security for them so they can come to work and have a job."
Rocky Lucia, the union's attorney, said, "Basically we bought labor peace."
When the Oakland City Council voted last week to approve the city's budget for the new fiscal year, several council members said Oakland police officers have larger salaries and more benefits than most of their colleagues in nearby cities, even with the concessions.
Arotzarena said he was not sure if that is true, but he said he hopes that is the case "because we earn our paychecks because we do more work than any other police department."
Arotzarena said Oakland had more than 800 officers two years ago and now that the total has shrunk to 636 officers, each remaining officer must do more work.
"We're very much overworked," he said, saying Oakland has the highest violent crime rate of any city in California and is the fifth most dangerous city in the nation, according to some studies.
Members of Oakland's other major city employee unions are also voting on concessions that their leaderships tentatively agreed upon. The final results of all those votes won't be disclosed until Friday.
If any of the unions reject their proposed concessions, the City Council will be forced to start over again on the budget that was just approved last week.