O.Thetford, P.Gray German Aircraft of the First World War (Putnam)

Otto C I (1915)
  A disused airframe on public display. Note kink in ailerons to give wash-out, as on L.V.G. machines. Engines, 150 h.p. Benz Bz III. Span, 14.5 m. (47 ft. 7 in.). Weights: Empty, 915 kg. (2,013 lb.). Loaded, 1,500 kg. (3,300 lb.). Armament, one Parabellum machine-gun in front cockpit.

J.Herris Otto, AGO and BFW Aircraft of WWI (A Centennial Perspective on Great War Airplanes 37)

Otto C.I

  Whereas many operational Allied aircraft were pushers, the Otto C.I was one of the very few German aircraft to use that configuration. A partial explanation for this is because the Otto company was located in Munich, Bavaria, and the Bavarian air service, which purchased Otto aircraft, was independent of the Prussian-controlled German air service up until about 1915. The German air service strongly preferred tractor aircraft and most production German two-seaters were of that configuration, but, like France and Britain, the Bavarian air service initially favored pusher aircraft. To the Bavarians, the chief benefit of pusher two-seaters was the unobstructed forward visibility for the crew for observation, photography, and bombing.
  Consequently, when in 1912 the Bavarian air service started purchasing aircraft in earnest, 17 Otto pushers (unarmed two-seater biplanes) and six Euler pushers were obtained. The primitive and underpowered Otto B-types with which the Bavarians started combat operations in August 1914 were replaced with LVG and Albatros B-type biplanes starting in December 1914. Thus, in early 1915 the aircraft destined to become the Otto C.I was begun as a private venture to replace the obsolete Otto B-types.
  The prototype of the new aircraft, known as the Otto KD 15, performed its first flight in late May or early June 1915 powered by a 165 hp Austro-Daimler six-cylinder engine. It had twin booms to reduce drag compared to the Otto B and its wing planform and cranked ailerons were based on that of the LVG B-type biplanes then being produced by Otto under license. The prototype KD 15, later to become known as the Otto C.I, was totally constructed of Duralumin, quite an innovation for the time, which saved 200 kg or more of weight compared to steel tubing. However, since Duralumin could not be welded or soldered, the joints had to be connected with bolted sleeve clamps. A four-wheeled undercarriage was mounted to help prevent nose-overs on rough fields and, unlike most aircraft of the time, the rear wheels had brakes.
  The second Otto KD 15 was initially flown with a larger wing than the first prototype, but after being damaged in a hard landing on 7 July 1915, it was rebuilt with wings of reduced span in August. The second prototype also featured a 200 hp Rapp engine and wooden fairings on its metal struts to reduce drag.
  Because the Otto firm was financially distressed, Gustav Otto, the owner, requested and received a production order for six Otto C.I aircraft in August. These six aircraft, designated Otto C.I 544/15 to 549/15, were followed by an order for an additional six aircraft, designated C.I 1115/15 to 1120/15, in November. In addition, the first prototype was purchased after the initial production order had been placed and was given number C.I 3633/15, and the second prototype may also have been purchased for use.
  Production Otto C.I aircraft were powered by a variety of engines, including the 200 hp Rapp, the 160 hp Mercedes, and the 150 hp Benz. Despite its higher power rating, the Rapp was unpopular because it was not nearly as reliable as the Mercedes or Benz, which were strongly preferred for service use. Rapp, located in Bavaria, was later purchased by new owners and reorganized as the Bavarian Motor Works, which then produced the excellent 185 hp BMW.III engine of 1918.
  Easy to fly but with a limited operational ceiling of only about 3,000 metres (9,800 feet) when fully equipped, in German service the Otto C.I served only with Bavarian units such as Flieger-Abteilung 4b, 8b, and 9b, and was soon relegated to training units, where the remaining aircraft were converted to dual controls for pilot training.

Bulgarian Service

  The Otto C.I first batch served with Bavarian Flieger-Abteilungs and one with Prussian FFA58 before it was converted to training service. In 1915 a Bulgarian purchasing commission visited Munich and were taken to see the Otto Werke, where about a dozen Otto C.I aircraft were in storage. The aircraft were available immediately and the Bulgarians bought them all.
  In Bulgarian service the Otto C.I aircraft seemed to be used in action only by the 2nd Aeroplane Otdelenie based at Udovo and others were at the Aeroplane School at Bojurishte.
  Thirteen Otto C.I two-seat twin-boomed biplanes (powered by the 150 hp Benz Bz.III) were delivered to Bojurishte in the spring of 1916. They were used by 2 Aeroplane Otdelenie at Udovo airfield in the bombing and reconnaissance roles. These were still used at the start of 1917 for reconnaissance but soon withdrawn in favor of the Albatros C.III. The known serial numbers appear in the nearby table.

Otto C.I Aircraft Production
  Production Serials Qty Notes
  Prototype C.3633/15 1 At Schleifiheim school (2nd prototype serial unknown)
  1st batch C.544/15-549/15 6 Primarily used by Bavaria (mostly FFA 2b, FFA 4b, FFA 9b, Si FFA 58)
  2nd batch C.1115/15-1120/15 6 Bulgaria
  3rd batch C.3575/15-3580/15 6 Bulgaria

Bulgarian Otto C.I Aircraft
  Serial Delivered Notes:
  C.1115/15 1916 Shown at 2 Otdelenie. Suffered minor crash but appears to have been repaired.
  C.3575/15 1916
  C.3579/15 1916
  C.3580/15 1916
  Probably all Bulgarian Otto C.I aircraft were from production series 2 & 3.