O'Hare International Airport , also known simply as O'Hare Airport or O'Hare, is an airport located in the northwestern-most corner of Chicago, Illinois, United States, 17 miles (27 km) northwest of the Chicago Loop. It is the largest hub of United Airlines (whose headquarters is in downtown Chicago) and the second-largest hub of American Airlines (after Dallas/Fort Worth). It is operated by the City of Chicago Department of Aviation, associated with an umbrella regional authority.

In 2005, the airport had 972,246 aircraft operations, an average of 2,663 per day (64% scheduled commercial, 33% air taxi, 3% general aviation and Prior to 2005, O'Hare was the world's busiest airport in terms of takeoffs and landings. That year, mainly due to limits imposed by the federal government to reduce flight delays at O'Hare, Hartsfield-Jackson Atlanta International Airport became the busiest by that metric. O'Hare currently accounts for over a sixth of the nation's total flight cancellations. Currently, O'Hare International Airport is the second busiest airport in the United States in terms of traffic, and the second in the world with 76,248,911 passengers passing through the airport in 2006; a -0.3% change from 2005. O'Hare also has a strong international presence, with flights to more than 60 foreign destinations. O'Hare was ranked fourth in 2005 of the United States' international gateways, with only John F. Kennedy International Airport in New York City, Los Angeles International Airport, and Miami International Airport serving more foreign destinations.

O’Hare International Airport was voted the Best Airport in North America for the past nine years by readers of the U.S. Edition of Business Traveler Magazine (1998 - 2003), and Global Traveler Magazine (2004 - 2007), marking the tenth year in a row O'Hare has earned the top honor.

Although O'Hare is Chicago's chief airport, Chicago Midway International Airport, the city's second airport, is about six miles closer to the Loop, the main business and financial district. Located 60 miles northwest of O'Hare is Chicago's third regional airport, Chicago Rockford International Airport.