The Federal Aviation Administration says O'Hare experienced 17 errors within a one-year period, an increase from six errors the year before.
O'Hare is training new employees as part of the ramp-up to open the next air traffic tower to boost O'Hare's capacity.
Data show the errors happened while new controllers were directing aircraft to land, take off or taxi. The main error was not keeping proper distance between airplanes.
The FAA says it is working to reduce training fatigue by veteran controllers.
(The Associated Press contributed to this report.)
