The OC-135B Open Skies is a United States Air Force observation aircraft that supports the Treaty on Open Skies. The aircraft, a modified WC-135B, flies unarmed observation flights over participating parties of the treaty. Three OC-135B aircraft were modified by the Aeronautical Systems Center's 4950th Test Wing at Wright-Patterson Air Force Base in Ohio but only two were ever used. The first operationally-capable OC-135B was assigned to the 24th Reconnaissance Squadron at Offutt AFB in October 1993. It is now fitted with a basic set of navigational and sensor equipment, and was placed in inviolate storage at the Aerospace Maintenance and Regeneration Center at Davis-Monthan Air Force Base near Tucson, Arizona in 1997. Two fully operational OC-135B aircraft were delivered in 1996 with the full complement of treaty-allowed sensors, which includes an infrared line scanner, synthetic aperture radar and video scanning sensors.

The Treaty on Open Skies establishes a program of unarmed aerial surveillance flights over the entire territory of its participants. The treaty is designed to enhance mutual understanding and confidence by giving all participants, regardless of size, a direct role in gathering information about military forces and activities of concern to them. It entered into force on 1 January 2002, and currently has 34 party states. The idea of allowing countries to openly surveil each other is thought to prevent misunderstandings and limit the escalation of tensions. It also provides mutual accountability for countries to follow through on treaty promises.