The shooting happened around 6:30 p.m. Monday on the 1700 block of Seminary Avenue near International Boulevard. Police say the officer was working undercover at the time, investigating another shooting that happened on Sunday.
Officials say they've detained one man but are still searching for two others believed to be armed and dangerous in the neighborhood just east of the Coliseum.
SWAT members along with police officers and K-9 units closed down several streets as they conducted yard-to-yard searches.
Police say it all started when the undercover officer was confronted by three men on Seminary Avenue.
When asked how many officers were present and if they returned fire, Oakland Police Chief Howard Jordan answered, "Unknown at this time if the officer returned fire. There was one officer involved in this incident. We believe he was in his car, that's as much as we know right now."
Police aren't saying what exactly triggered that shooting.
A perimeter was set up and access to the East Oakland neighborhood area was restricted Monday as police continued their investigation. Some neighbors complained because they weren't allowed in the perimeter. But police say they just couldn't take any chances.
The officer was shot in the arm and transported to a local hospital. His injuries are considered serious but not life-threatening.
Oakland police have asked residents in the area who may have seen something suspicious to call 911 or (510) 777-3211.