All four sister ships were assigned to the 2nd Battle Squadron of the Home Fleet upon commission and "Orion" was named its flagship. The sisters then participated in the Fleet review at Spithead on 9 July 1914 and in the mobilisation of the Royal Navy during the July Crisis and following assembly at Scapa Flow. The sisters were four of the six dreadnoughts assigned to intercept the German fleet responsible for the 16 December raid on the northeast English coast. On 27 December 1914, "Conqueror" rammed her sister ship "Monarch" as the Grand Fleet was returning to Scapa Flow, damaging both vessels. All four warships were present at the Battle of Jutland. "Conqueror" and "Thunderer" made no hits, while "Monarch" and "Orion" struck and once each and five times between them; none of the four fired more than 57 of their primary shells during the entire battle. When on 21 November 1918 the German fleet surrendered at Rosyth, Scotland, the sisters were present. In February 1919, they were transferred to the 3rd Squadron and "Orion" retained her status as squadron flagship. By the end of 1919 the sisters were all transferred into the Reserve Fleet, but "Monarch" was transferred to Portsmouth in early 1920. In the summer of that year, "Thunderer" and "Monarch" were recommissioned to ferry troops to and from the Mediterranean Sea, and "Orion" joined "Monarch" at Portsmouth later in the year and became the flagship of the Reserve Fleet before being relieved in this duty by "Conqueror" in mid-1921 to once again ferry troops. In 1921, "Thunderer" and "Orion" were transformed into training vessels and were sold for scrap the following year in accordance with the Washington Naval Treaty. "Monarch" was hulked and used for weapons testing until finally sunk in 1925. "Thunderer", the last of the sisters, was sold for scrap in 1926.