The school didn't reveal contents of the threats. But university Police Chief Sam Lucido told the Detroit Free Press that they referred to possible campus attacks on "4/14."
That is the same date noted in threatening graffiti found twice this month in a freshman dorm at St. Xavier University in the Chicago area. Officials at the Catholic liberal arts college shut the school down indefinitely on Friday and told students to leave its campuses in Chicago and suburban Orland Park, Ill.
Lucido spoke Sunday with the head of security at St. Xavier, Oakland spokesman Ted Montgomery told The Associated Press.
"I don't think as of yet they've established any connection that seems reliable," he said.
Lucido told the Free Press that the Oakland University threats didn't target anyone specific, and that authorities believe the same person left all three threats.
The activities cancellation was in effect at the school for Sunday and Monday. Dormitories remained open, although Oakland University spokeswoman Michelle Moser said students were encouraged to go home if possible.
Campus officials "are taking this threat seriously and are closing to ensure the safety of the entire campus community," Lucido said.
The public university has about 18,000 students in Rochester, about 20 miles north of Detroit.
St. Xavier spokesman Joe Moore said Sunday that students would be informed by e-mail, phone text messages and a school Web site of any decision to reopen.
"We're hoping to open as soon as possible," Moore said.
Campuses around Illinois have been on alert since a Feb. 14 shooting in which a gunman burst into a lecture hall at Northern Illinois University, killing five students and wounding 18 others before turning the gun on himself.