Chicago O'Hare International Airport opened for commercial service on October 29, 1955, when a dedication ceremony drew 23,000 people to watch TWA Flight 94 depart for Paris. Scheduled service began the following day, with American Airlines operating the first scheduled arrival and departure. The airport had begun life as a wartime Douglas manufacturing plant, and its IATA code, ORD, survives from its original name, Orchard Field. It was renamed in 1949 to honor Edward "Butch" O'Hare, the U.S. Navy's first Medal of Honor recipient in World War II.

Growth was slow at first, with airlines reluctant to leave Midway, then the world's busiest airport. That changed with the jet age. Midway had no room for the runways that Boeing 707s and Douglas DC-8s required, and by July 1962 all scheduled Chicago flights had transferred to ORD. Passenger numbers immediately reached 10 million, making O'Hare the world's busiest airport, a title it held until 1998, when Hartsfield-Jackson Atlanta surpassed it. After airline deregulation in 1978, American Airlines and United Airlines established dual hubs here, a combination that remains one of the few of its kind among major U.S. airports.

ORD sits approximately 17 miles northwest of downtown Chicago, straddling Cook and DuPage counties across more than 7,627 acres. Its 8 runways, more than any other airport in the world, are the product of a major modernization program announced in 2001 and completed in 2021, which replaced an inefficient intersecting layout with parallel configurations. The airport's 4 terminals serve more than 249 destinations across 6 continents. Terminal 1, opened in 1987 and designed by architect Helmut Jahn, is United Airlines' primary hub; Terminal 5, opened in 1993, handles most international arrivals. The CTA Blue Line connects the airport directly to downtown Chicago.