The Orion class battleships were four super-dreadnoughts — the first ships of that type — of the Royal Navy's. The lead ship, , was launched in 1910. They were the first Royal Navy dreadnoughts to have all their main guns in the centerline, although the U.S. South Carolina-class had this advanced feature in their first dreadnoughts. The Orions were quite distinct from the later classes of 13.5" super-dreadnoughts (King George Vs and Iron Dukes) in that the fore-funnel was placed in front of the mainmast. This arrangement was common in early RN Dreadnoughts but created major problems for the gunnery direction team in the spotting top.
The move to the gun came out of necessity. The final iteration of the , the high velocity 50 calibre Mark XI, had been unsuccessful. It lost some of its accuracy and was subject to considerable bore erosion that limited its useful life to about 80 discharges. A lower velocity weapon was indicated, and was realized in the 13.5-inch, of 45 calibres and with a much heavier projectile. The projectile used was or against . The shorter range of the low-velocity gun was compensated by increasing the maximum elevation from 15 to 20 degrees. The 13.5-inch was considered a successful design, although its effectiveness was compromised by poor shell design until availability of the "Green Boy" shells in 1918.
The adoption of the all-centreline armament was also of necessity. The previous Colossus- and Neptune-classes had already adopted a superfiring pair of turrets aft in an effort to save deck space, and competing designs in the U.S. and elsewhere had led the way in the all-centreline arrangement. The great investment in displacement and cost of a 13.5-inch turret contra-indicated wing turrets, with their limited arcs of fire and therefore limited effectiveness. Finally, the staggered arrangement of the Colossuses and Neptunes complicated internal arrangements and was generally not considered a success. Unfortunately, the retention of outdated sighting hoods meant the Orions were not able to fire the superimposed turrets over the deck turrets for fear that the blast would injure crews in the lower turrets. Also, the design's relatively high topweight gave the Orions a wicked roll in rough seas. Both these defects were eventually cured and the improvements included in subsequent classes of British Dreadnought.
The armour belt was increased to in view of the general increase in gun calibre in potential adversaries. It is quite typical of British designs armour thickness lagged behind gun size, this being the opposite design driver of German ships in which armour thickness was usually greater than the calibre of the guns they carried. It is also of interest to note adequacy of ship armour was still being judged by belt (vertical) thickness, rather than deck (horizontal) thickness which became more critical as battle ranges extended beyond approximately 12,000 yards (11 km) as the trajectory of the incoming shells was more vertical than the design of the ships was intended to resist. Practice ranges prior to the war were rarely greater than at which the trajectory was nearly flat. By the time that the initial actions were being fought at ranges exceeding on occasions all capital ships on both sides were vulnerable to "plunging" fire. (German ships were less vulnerable by virtue of having very deep belts, which kept shells away from the armour decks, and because British shells were notoriously weak at penetrating armour at an oblique angle).
The Orions cost almost £1.9 million to construct. All four were present at the Battle of Jutland of 1916 in World War I, but took no damage. They had a relatively short career, all being decommissioned in 1921, due to the Washington Naval Treaty. Orion and Conqueror were scrapped in 1922. Monarch served as a target, surviving a full day of shelling and bombing on 20 January 1925 before being sunk by fire from . Thunderer served longest, acting as a training ship from 1922 until she, too, was sold for scrap in December 1926.