The Cessna O-2 was a militarised version of the Cessna 337 Skymaster six-seat light aircraft. The aircraft was first ordered into production for the USAF in December 1966 and was adapted to carry marker rockets and other ordnance for the role of forward air control. The first O-2As arrived in South Vietnam on 9 June with the intention that the type would eventually replace the older and smaller O-1 Bird Dog. Most were assigned to the 20th TASS and the 23rd TASS as these squadrons encountered the heaviest opposition. The 19th, 21st and 22nd TASS also received O-2s but still retained a large number of O-1s. Indeed, by January 1968 there were still 181 USAF O-1s based in Southeast Asia compared to 137 O-2s. Although the O-2 performed well in Southeast Asia it never entirely replaced the older aircraft. The O-2 could not operate from some of the smaller airstrips that the O-1 could fly into and visibility from the O-2 was not as good as that from the Bird Dog. The O-2 certainly had a better performance than the O-1 including the added safety that comes from two engines, especially as the engines were situated on the aircraft’s centreline so that the failure of one engine did not produce a marked assymetrical flight condition. Many pilots never really warmed to the O-2 and it was regarded by most as a stopgap aircraft until the introduction of the specialist OV-10 Bronco in the following year.