For at least an hour after the shooting began, police thought the shooter could still be on campus.
Myung Soon Ma, the school's secretary, said she could not provide any details about what happened at the small private school, 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 they 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.
Officer Johnna Watson said the suspect is an Asian male in his 40s who was taken into custody at a shopping center in the neighboring city of Alameda. She would not confirm if he was a student.
Watson said most of the wounded or dead were shot inside the building.
"It's a very fluid situation and an active investigation," Watson said, declining to discuss details of the arrest or a possible motive.
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.
Jerry Sung, the university's accountant, said the school offers courses in both Korean and English to less than 100 students. He said the campus consisted of one building.
Sung said many of its students went on to work in nursing and ministry.
"The founder felt there was a need for theology and nursing courses for Korean-Americans who were newer to the community," Sung said. "He felt they would feed more comfortable if they had Korean-American professors."