Production deliveries of the O/400 began in early 1918. This was an improved version of the O/100 which differed primarily by having more powerful Rolls-Royce Eagle engines, a revised fuel system and radiators, and the introduction of a compressed-air engine starting system. Although production of O/100s totalled only 46 aircraft, substantial numbers of O/400s became operational before the end of the war. An example of this was on the night of 14th-15th September 1918 when a force of 40 O/400s attacked targets in the Saar. It was also at about this time that these aircraft began to deploy 1,650 lb bombs, the heaviest used by British services during World War 1.
More than 400 O/400s were delivered for service with the RAF before the Armistice of November 1918, these equipped Nos 58, 97, 115, 207, 214, 215 and 216 Squadrons. The type remained in service in reduced numbers until late 1919, when it was replaced by the Vickers Vimy. Eight of these aircraft were used by No. 86 (Communications) Wing, formed at Hendon in December 1918 to provide government and military VIP transport between London and Paris, and No. 214 Squadron used the type for military airmail services.
It had been planned to build the type in quantity in the United States, but only 107 Liberty-engined aircraft were completed by the Standard Aircraft Corporation before the armistice brought contract cancellation of the balance of 1,500 that were on order. Post-war, a small number of British-built aircraft was supplied to China under the designation O/7, and three or four of these aircraft were used in India by Handley Page Indo-Burmese Transport Ltd. In addition, about 10 O/400s were converted from military to a civil configuration and used by Handley Page Transport Ltd in the UK with the designations O/10 and O/11.
In the 1924 system the O/400 became the H.P.12.