Chicago O'Hare International Airport (ORD) officially opened for commercial airline service on October 29, 1955, when a TWA flight departed for Paris before a crowd of 23,000 at the dedication ceremony. The airport's IATA code, ORD, is a relic of an earlier identity: the site began as a Douglas manufacturing plant during World War II, was renamed Orchard Field Airport, and in 1949 was rechristened in honor of Lieutenant Commander Edward "Butch" O'Hare, the U.S. Navy's first Medal of Honor recipient of the war.

For its first years, growth was slow. Midway Airport held most of Chicago's traffic. That changed abruptly in 1959, when jets arrived and Midway's shorter runways could not accommodate them. Airlines relocated en masse. By 1962, all scheduled Midway operations had transferred to ORD, and O'Hare's passenger count hit 10 million, making it the world's busiest airport virtually overnight. It held that distinction by passenger volume from 1963 to 1998. Today it ranks as the world's busiest airport by aircraft movements, recording more than 857,000 operations in 2025.

Located 17 miles northwest of downtown Chicago and accessible via the CTA Blue Line, O'Hare covers more than 7,600 acres and operates 8 runways, more than any other airport on earth. The current parallel runway configuration is the product of an $8 billion modernization program announced in 2001, which replaced a legacy layout of intersecting runways that had caused cascading delays across the national airspace. United Airlines anchors Terminal 1, American Airlines operates from Terminal 3, and the airport collectively offers non-stop service to 249 destinations across six continents. O'Hare pioneered the concourse design, jet bridges, and underground fueling systems that are now standard at major airports worldwide.