Emergency teams from more than 30 agencies took part in a simulated full-scale emergency exercise at the Chicago airport.
The city's fire and police departments took part in the drill, which tested the mobilization, communications and coordination between all the agencies involved.
Hundreds of volunteers were also present posing as injured airline passengers.
The exercise was scheduled for 10 a.m. Saturday at the airport's Aviation Administration Building, 10510 W. Zemke Rd.
Airports must conduct emergency testing every three years as part of a FAA requirement, officials said.
O'Hare's last exercise took place in September 2014.