The power went off just before 4 p.m. at Terminals 2, 3 and 5. It was restored at about 4:20 p.m.
According to the Department Aviaton, flights were unaffected. However, there were some delays as a result of the outage.
Even after power was restored to most of the airport, baggage carrier belts remained without electricity, causing a major backup.
"They told us that it should be out in 15 minutes," said traveler Santiago Guerrero, who had been waiting for two and half hours. He later got his bags.
Sean Crosby was on a plane sitting at a gate when the terminal went dark.
"I didn't want to move it, which I guess was a smart idea," said Crosby.
"There was never any kind of safety issue that was compromised because of the power outage," said Karen Pride, City Aviation Department.
ComEd sent crews to O'Hare in the wake of the outage. A spokeswoman says the cause of the outage hasn't been determined.
"We have crews in the field doing a full 24/7 investigation. And once we find out what the cause is, we'll be able to say more. But at this time we don't know what the cause was," said Mike McMahan, ComEd.
O'Hare was not alone. New Jersey's Newark Airport lost power Monday afternoon for about 90 minutes.
The Associated Press contributed to this report.