Blue Line stations Downtown include La Salle, Jackson, Monroe and Washington. The O'Hare CTA station is located on the lowest level of the Parking Garage in the main terminal area. Passengers for Terminal 5 go to Terminal 3 and take the ATS transfer train to Terminal 5. Journey time from Downtown to O'Hare is around 45 minutes.
Trains run every ten minutes during the day.
More information is available at the Chicago Transit Authority web site . The Blue Line train is a cheap and efficient way to O'Hare from Downtown Chicago. For safety it's better not to use the train late at night.