Osan Air Base (IATA: OSN, ICAO: RKSO) (K-55; Korean: 오산공군기지; Hanja: 烏山空軍基地) is a United States Air Force (USAF) and Republic of Korea Air Force (ROKAF) base located near Songtan Station in the city of Pyeongtaek, South Korea, 64 km (40 mi) south of Seoul. Despite its name, Osan AB is not within Osan City, which is 7.5 km (4.7 mi) to the north. The base is the home of the headquarters for Seventh Air Force, Pacific Air Forces' 51st Fighter Wing, and a number of tenant units. The base is also the headquarters of the Republic of Korea Air Force (ROKAF) Operations Command. Osan Air Base is also the departure and arrival point for U.S. government-contracted "Patriot Express" flights bringing service members and their family members to South Korea from Seattle-Tacoma International Airport in the U.S. state of Washington, Misawa Air Base and Yokota Air Base in Japan.

As the most forward deployed permanently based wing in the Air Force, and equipped with A-10 Thunderbolt IIs and F-16 Fighting Falcons (about 48 aircraft), the 51st Fighter Wing is charged with executing combat operations, receiving follow-on forces and defending the base from attack. As the air component to United States Forces Korea and Combined Forces Command, 7th Air Force provides the command and control structures and personnel necessary to deliver precise, persistent, combined air and space power in defense of the Republic of Korea.

Osan Air Base is one of two major U.S. Air Force installations operated by U.S. Forces Korea, the other being Kunsan Air Base.