The first 13.5-inch-gunned (343 mm) battleships built for the RN, they were much larger than the preceding British dreadnoughts and were sometimes termed "super-dreadnoughts".

The sister ships spent most of their careers assigned to the 2nd Battle Squadron of the Home and Grand Fleets, sometimes serving as flagships.

Aside from participating in the failed attempt to intercept the German ships that had bombarded Scarborough, Hartlepool and Whitby in late 1914, the Battle of Jutland in May 1916 and the inconclusive action of 19 August, their service during World War I generally consisted of routine patrols and training in the North Sea.

The idea had been pioneered by the United States Navy in their South Carolina class, but the RN was slow to adopt the concept, concerned about the effects of muzzle blast on the gunlayers in the open sighting hoods in the roofs of the lower turrets.

[2] One of the few things retained from earlier ships was the position of the tripod foremast with its spotting top behind the forward funnel to allow the vertical leg to be used to support the boat-handling derrick.

This all but guaranteed that the hot funnel gases could render the spotting top uninhabitable at times, but the Board of Admiralty insisted on it for all the ships of the 1909–1910 Programme.

[3] David K. Brown, naval architect and historian, commented on the whole fiasco: "It is amazing that it took so long to attain a satisfactory arrangement, which was caused by the Director of Naval Ordnance (DNO)'s insistence on sighting hoods in the roofs of turrets and Jellicoe's obsession with boat-handling arrangements.

They were rated at 27,000 shaft horsepower (20,000 kW) and were intended to give the battleships a maximum speed of 21 knots (39 km/h; 24 mph).

This gave them a range of 6,730 nautical miles (12,460 km; 7,740 mi) at a cruising speed of 10 knots (19 km/h; 12 mph).

[7] The Orion class was equipped with ten 45-calibre Mark V guns in five hydraulically powered, centreline, twin-gun turrets, designated 'A', 'B', 'Q', 'X' and 'Y' from front to rear.

[5] Their secondary armament consisted of sixteen 50-calibre BL four-inch (102 mm) Mark VII guns.

[17] Furthermore, the ships were fitted with Mark II or III Dreyer Fire-control Tables, in early 1914, in each transmission station.

The forward oblique 6-inch (152 mm) bulkheads connected the waterline and upper armour belts to the 'A' barbette.

Like the Colossus-class ships, the Orions eliminated the anti-torpedo bulkheads that protected the engine and boiler rooms, reverting to the scheme in the older dreadnoughts that placed them only outboard of the magazines with thicknesses ranging from 1 to 1.75 inches (25 to 44 mm).

One or two flying-off platforms were fitted aboard each ship during 1917–1918; these were mounted on turret roofs and extended onto the gun barrels.

They then participated in training manoeuvres with Vice-Admiral Prince Louis of Battenberg commanding the "Blue Fleet" aboard Thunderer.

During the annual manoeuvres in August, Thunderer was the flagship of Vice-Admiral Sir John Jellicoe, commander of the "Red Fleet".

This grand battle was slow to happen, however, because of the Germans' reluctance to commit their battleships against the superior British force.

The radio messages did not mention that the High Seas Fleet with fourteen dreadnoughts and eight pre-dreadnoughts would reinforce Hipper.

The Germans got the better of the initial exchange of fire, severely damaging several British destroyers but Admiral Friedrich von Ingenohl, commander of the High Seas Fleet, ordered his ships to turn away, concerned about the possibility of a massed attack by British destroyers in the dawn.

[27] On 27 December, Conqueror accidentally rammed Monarch as the Grand Fleet was returning to Scapa Flow in heavy weather and poor visibility.

[29] Once Jellicoe's ships had rendezvoused with the 2nd BS, coming from Cromarty, Scotland, on the morning of 31 May, he organised the main body of the Grand Fleet in parallel columns of divisions of four dreadnoughts each.

[30] In the early stage of the battle, Conqueror and Thunderer fired at the crippled light cruiser SMS Wiesbaden with unknown results.

[38] Conqueror followed Orion to Portsmouth and relieved her as flagship in mid-1921 and the latter ship was again temporarily recommissioned to transport troops.

In accordance with the terms of the Washington Naval Treaty, Orion and Conqueror were sold for scrap in 1922 and broken up the following year; Monarch was initially listed for sale, but was hulked instead.

On her way to the scrapyard, she ran aground at the entrance to the Port of Blyth, Northumberland, but was refloated and scrapped the following year.