Myung Soon Ma, the school's secretary, said she could not provide any details about what happened at the university, which serves the Korean community with courses from theology to Asian medicine.
"I feel really sad, so I cannot talk right now," she said, speaking from her home. "No one can go there because the access is restricted right now."
Police believe the shooter acted alone, though have not discussed a possible motive.
Police spokeswoman Cynthia Perkins said the death toll was seven Monday afternoon. She did not release any other details about the victims, but said officials planned a news conference later.
The suspect was taken into custody in the neighboring city of Alameda, according to law enforcement officials who spoke on condition of anonymity because they were not authorized to release the information.
Officer Johnna Watson did not give any other details about the suspect. Authorities earlier described him as a heavyset Korean man in his 40s wearing khaki clothing.
KTVU-TV reported that the shooter was a student and opened fire in a classroom.
Pastor Jong Kim, who founded the school about 10 years ago, told the Oakland Tribune that the shooter was a nursing student who was no longer enrolled. He did not know if the shooter was expelled or dropped out.
Kim said he heard about 30 rapid-fire gunshots in the building.
"I stayed in my office," he said.
Deborah Lee, who was in an English language class, said she heard five to six gunshots at first. "The teacher said, 'Run,' and we run," she said. "I was OK, because I know God protects me. I'm not afraid of him."
Angie Johnson told the San Francisco Chronicle that she saw a young woman leave the building with blood coming from her arm and crying: "I've been shot. I've been shot."
The injured woman said the shooter was a man in her nursing class who got up and shot one person at point-blank range in the chest before spraying the room with bullets, Johnson said.
"She said he looked crazy all the time," she said the victim told her, "but they never knew how far he would go."
According to its website, Oikos University also offers studies in music and nursing. A telephone message left on the university's main voicemail was not immediately returned.