||Primary user|| Produced
The important family of Douglas observation aircraft sprang from two XO-2 prototypes, the first of which was powered by the 420 hp (313 kW) Liberty V-1650-1 V-engine and test-flown in the autumn of 1924. The second XO-2 was powered by the 510 hp (380 kW) Packard 1A-1500 Vee engine, which proved unreliable. The US Army ordered 45 O-2 production aircraft in 1925, these retaining the XO-2's welded steel tube fuselage, wooden wings and overall fabric covering but at the same time introducing aluminum panels on the forward fuselage. The XO-2 had been flown with short and long-span wings, the latter giving improved handling and therefore being specified for the production aircraft. The fixed tailskid landing gear included a main unit of the divided type, the horizontal tail surface was strut braced, and the engine was cooled by a tunnel radiator.
The O-2 proved to be a conventional but very reliable biplane which soon attracted orders for 25 more aircraft: 18 O-2A machines equipped for night flying and six O-2B dual-control command aircraft for the US Army, plus one civil O-2BS modified specially for James McKee, who made a remarkable trans-Canada flight in September 1926. In 1927 the O-2BS was adapted as a three-seater with a radial engine.
The O-2Hs were an entirely new design but continued the same basic model number. Major differences were heavily staggered wings, a more compact engine installation, and clean landing gear secured to the fuselage.
Up to 2011 there were no O-2's known to exist. However in 2011 the wreckage of O-2H 29-163 that crashed out of Kelly Field Texas on March 16, 1933 has been positively identified. The rear and central/forward portion of the fuselage behind the firewall, wing attachments and landing gear parts, tailplane and many engine parts and eight of the twelve pistons are now recovered. Research is continuing on this aircraft. It is known it was flown by Aviation Cadet Charles D. Rogers on a night recon advanced training mission. Apparently flying low, the aircraft hit a hill and burned after the crash leaving only the found wreckage today. Weather was not considered a contributing factor. Cadet Rogers was instantly killed in the crash by the impact. His body was recovered but the wreckage was abandoned due to the airframe and engine both being a writeoff.
The only similar aircraft known to exist are a restored Douglas M-2 mailplane and a follow-on derivative of the O-25 variant, an O-38.
- Two pre-production prototypes.
- Initial production model - 45 built.
- O-2 with night flying equipment - 18 built.
- Dual control version of O-2 - six built.
- These differed from the O-2 in having frontal radiators for their Liberty engines and modified oleo-strut landing gear. The USAAC took delivery of 18 aircraft, while the remaining 27 went to reserve National Guard units - 45 built and one later conversion from O-9.
- Unarmed staff transport versions of the O-2C - two built.
- A one-off aircraft which replaced the wire link between upper and lower wing ailerons of production aircraft by rigid struts.
- The fuselage was redesigned and a new tailplane was fitted, with staggered wings of unequal span. The O-2H incorporated the rigid-strut aileron interconnections of the O-2E. An improved split-axle landing gear was standard. The USAAC received 101 O-2Hs between 1928 and 1930, and the National Guard a further 40 - 141 built.
- Unarmed dual control version of the O-2H for service as USAAC staff transports - three built.
- A slightly modified version of the O-2J for US Army staff transport and liaison duties. 30 built for the USAAC and 20 for the National Guard - 50 built.
- various export versions of O-2 that saw services with Republic of China Air Force. These aircraft were used as scout-bombers by the Chinese in the Second Sino-Japanese War with somewhat limited success against Japanese ground targets. It was also used by the Mexican Air Force with Lewis and Vickers machine guns, with very good results.
- Export version for China, powered by a Hornet radial engine - ten built
- Export version for China, with the Hornet radial engine surrounded by a Townend ring - 20 built
- Export version for China, fitted with an uprated 429-kW (575-hp) Hornet radial engine - five built
- Export version for China - 12 built
- Export version for China, fitted with the less powerful 313-kW (420-hp) Pratt & Whitney Wasp C1 engine - 12 built
- Export version for China, fitted with the 429-kW (575-hp) Wright R-1820-E radial engine - 22 built
- Export version for China, fitted with a 500-kW (670-hp) Wright R-1820-F21 radial engine - one built
- Five all-metal O-2s, built in the mid-1920s by Thomas-Morse.
- Radically altered (smaller and lighter) version of the XO-6 - one built.
- Three O-2s refitted with the 510-hp (380 kW) Packard 2A-1500 direct-drive engine. Two were later converted to O-2 standards, and one to the O-2C standard.
- One O-2 with the 400 hp (298 kW) Curtiss R-1454 radial engine instead of the intended Packard inverted-Vee engine. It later became an O-2A.
- One O-2 refitted with the 500 hp (373 kW) Packard 3A-1500 geared engine. It resembled the 0-7 but had a four- rather than two-bladed propeller. It later became an O-2A.
- One reduced-scale O-2H, with a 220 hp (164 kW) Wright J-5 engine, and the first Douglas aircraft with wheel brakes.
- The 46th aircraft of the original O-2 contract was completed as an attack machine with the powerplant of one 420 hp (313 kW) V-1410 Liberty inverted-Vee engine, and with a total of eight machine-guns (two in the engine cowling, two each in the upper and lower wings, and two on a ring-mounting operated by the observer). It was remarkably well armed for its day, and competed against the Curtiss A-3 in 1926 but was not selected for production.
- Two O-2Cs for service with the US Marine Corps from 1929.
- O-2H airframe with a swept-back upper wing and a Pratt & Whitney Wasp engine.
- O-2H airframe with a Curtiss Conqueror engine, and a revised nose. Later redesignated as the XO-25A 
- Forty-nine production versions of the O-25.
- Three unarmed O-25As fitted with dual controls. Used as staff transport aircraft
- 29 production O-25s with Prestone cooling system
- Later designated O-29A: Two O-2K airframes fitted with a Wright R-1750 Cyclone engine.
- O-2K conversion with Pratt & Whitney R-1340-3 Wasp engine, most later fitted with anti-drag rings.
- Production O-32, 30 built.
- O-22 re-fitted with a Curtiss Conqueror engine.
- O-2K conversion to basic trainer, 30 converted.
- O-32 airframe converted to basic trainer.
- O-32A conversion to basic trainer, 30 converted.
- First production model, 146 built. 58 later converted to BT-2BI instrument trainers. Two converted to BT-2BR and 15 to BT-2BG radio-controlled aerial target drones.
- Second production model, 20 built. 13 converted to BT-2CI instrument trainers. Seven became BT-2CR drone controllers.
- Seventeen BT-2BRs and BT-2BGs converted in 1940 as radio-controlled aerial target drones. These had tricycle gear (a steerable nosewheel was added) with main gear moved aft, faired-over rear cockpit, and single controls, allowing the aircraft to be test-flown.
Data from Eden & Moeng (2002) p614 General characteristics
- Crew: two
- Length: 28 ft 9 in (8.76 m)
- Wingspan: 39 ft 8 in (12.09 m)
- Height: 10 ft 6 in (3.2 m)
- Wing area: 411 ft² (38.18 m²)
- Empty weight: 3,032 lb (1,375 kg)
- Max. takeoff weight: 4,785 lb (2,170 kg)
- Powerplant: 1 × V-1650 Liberty V-12 piston engine, 420 hp (313 kW)
- Maximum speed: 128 mph (206 km/h)
- Cruise speed: 103 mph (166 km/h)
- Range: 360 mi (579 km)
- Service ceiling: 16,279 ft (4,960 m)
- Rate of climb: 807 ft/min (246 m/min)</ul>Armament
- 2 × .30 in (7.62 mm) Browning machine guns, one fixed forward-firing and one flexible
- 400 lb (181 kg) of disposable stores carried under the lower wing
- Eden, Paul; Moeng, Soph (2002), The Complete Encyclopedia of World Aircraft, London: Amber Books, ISBN 978-0-7607-3432-2
- Swanborough, F. G.; Bowers, Peter M. (1964), United States Military Aircraft Since 1909, New York: Putnam, ISBN 0-85177-816-X
- Francillon, Rene. McDonnell Douglas Aircraft since 1920 (Putnam, 1979), p.89.
- Home Video of flying K model (around 6 min)