Chicago O'Hare International Airport (IATA: ORD, ICAO: KORD, FAA LID: ORD) is a major international airport serving Chicago, Illinois, United States, located on the city's Northwest Side, approximately 17 miles (27 km) northwest of the Loop business district. Operated by the Chicago Department of Aviation and covering 7,627 acres (11.92 sq mi; 30.87 km2). O'Hare has non-stop flights to 249 destinations in North America, South America, the Caribbean, Europe, Africa, Asia, the Middle East and the North Atlantic region as of Summer 2024. As of 2024, O'Hare is considered the most connected airport in the US, and 5th most connected airport in the world. It is also the United States' 4th busiest airport, and 7th largest airport.

Designed to be the successor to Chicago's Midway International Airport, itself once nicknamed the "busiest square mile in the world," O'Hare began as an airfield serving a Douglas manufacturing plant for C-54 military transports during World War II. It was renamed Orchard Field Airport in the mid-1940s and assigned the IATA code ORD. In 1949, it was renamed after aviator Edward "Butch" O'Hare, the U.S. Navy's first Medal of Honor recipient during that war. As the first major airport planned after World War II, O'Hare's innovative design pioneered concepts such as concourses, direct highway access to the terminal, jet bridges, and underground refueling systems.

O'Hare became famous during the jet age, holding the distinction as the world's busiest airport by passenger traffic from 1963 to 1998. It still ranks as one of the busiest airports in the world, according to the Airports Council International rankings.