The Revenge-class ships were designed as slightly smaller, slower, and more heavily protected versions of the preceding Queen Elizabeth-class battleships. As an economy measure they were intended to revert to the previous practice of using both fuel oil and coal, but First Sea Lord Jackie Fisher rescinded the decision for coal in October 1914. Still under construction, the ships were redesigned to employ oil-fired boilers that increased the power of the engines by 9,000 shaft horsepower (6,700 kW) over the original specification.
Royal Oak had a length overall of 620 feet 7 inches (189.2 m), a beam of 88 feet 6 inches (27.0 m) and a deep draught of 33 feet 7 inches (10.2 m). She had a designed displacement of 27,790 long tons (28,240 t) and displaced 31,130 long tons (31,630 t) at deep load. She was powered by two sets of Parsons steam turbines, each driving two shafts, using steam from 18 Yarrow boilers. The turbines were rated at 40,000 shp (30,000 kW) and intended to reach a maximum speed of 23 knots (42.6 km/h; 26.5 mph). During her sea trials on 22 May 1916, the ship only reached a top speed of 22 knots (41 km/h; 25 mph) from 40,360 shp (30,100 kW). She had a range of 7,000 nautical miles (12,964 km; 8,055 mi) at a cruising speed of 10 knots (18.5 km/h; 11.5 mph). Her crew numbered 909 officers and enlisted men in 1916.[
The Revenge class was equipped with eight breech-loading (BL) 15-inch (381 mm) Mk I guns in four twin gun turrets, in two superfiring pairs fore and aft of the superstructure, designated ‘A’, ‘B’, ‘X’, and ‘Y’ from front to rear. Twelve of the fourteen BL 6-inch (152 mm) Mk XII guns were mounted in casemates along the broadside of the vessel amidships; the remaining pair were mounted on the shelter deck and were protected by gun shields. Their anti-aircraft (AA) armament consisted of two quick-firing (QF) 3-inch (76 mm) 20 cwt Mk I] guns. The ships were fitted with four submerged 21-inch (533 mm) torpedo tubes, two on each broadside.[
Royal Oak was completed with two fire-control directors fitted with 15-foot (4.6 m) rangefinders. One was mounted above the conning tower, protected by an armoured hood, and the other was in the spotting top above the tripod foremast. Each turret was also fitted with a 15-foot rangefinder. The main armament could be controlled by ‘X’ turret as well. The secondary armament was primarily controlled by directors mounted on each side of the compass platform on the foremast once they were fitted in March 1917. A torpedo-control director with a 15-foot rangefinder was mounted at the aft end of the superstructure.[
The ship’s waterline belt consisted of Krupp cemented armour (KC) that was 13 inches (330 mm) thick between ‘A’ and ‘Y’ barbettes and thinned to 4 to 6 inches (102 to 152 mm) towards the ship’s ends, but did not reach either the bow or the stern. Above this was a strake of armour 6 inches thick that extended between ‘A’ and ‘X’ barbettes. Transverse bulkheads 4 to 6 inches thick ran at an angle from the ends of the thickest part of the waterline belt to ‘A’ and ‘Y’ barbettes. The gun turrets were protected by 11 to 13 inches (279 to 330 mm) of KC armour, except for the turret roofs which were 4.75–5 inches (121–127 mm) thick. The barbettes ranged in thickness from 6–10 inches (152–254 mm) above the upper deck, but were only 4 to 6 inches thick below it. The Revenge-class ships had multiple armoured decks that ranged from 1 to 4 inches (25 to 102 mm) in thickness. The main conning tower had 13 inches of armour on the sides with a 3-inch roof. The torpedo director in the rear superstructure had 6 inches of armour protecting it. After the Battle of Jutland, 1 inch of high-tensile steel was added to the main deck over the magazines and additional anti-flash equipment was added in the magazines.
The ship was fitted with flying-off platforms mounted on the roofs of ‘B’ and ‘X’ turrets in 1918, from which fighters and reconnaissance aircraft could launch. In 1934 the platforms were removed from the turrets and a catapult was installed on the roof of ‘X’ turret, along with a crane to recover a seaplane.[
Royal Oak was extensively refitted between 1922 and 1924, when her anti-aircraft defences were upgraded by replacing the original three-inch AA guns with a pair of QF four-inch (102 mm) Mk V AA guns. A 30-foot (9.1 m) rangefinder was fitted in ‘B’ turret and a simple high-angle rangefinder was added above the bridge. Underwater protection improved by the addition of anti-torpedo bulges. They were designed to reduce the effect of torpedo detonations and improve stability at the cost of widening the ship’s beam by over 13 feet (4 m). They increased her beam to 102 feet 1 inch (31.1 m), reduced her draught to 29 feet 6 inches (9.0 m), increased her metacentric height to 6.3 feet (1.9 m) at deep load, and all of the changes to her equipment increased her crew to a total of 1,188. Despite the bulges she was able to reach a speed of 21.75 knots (40.28 km/h; 25.03 mph). A brief refit in early 1927 saw the addition of two more four-inch AA guns and the removal of the six-inch guns from the shelter deck. About 1931, a High-Angle Control System (HACS) Mk I director replaced the high-angle rangefinder on the spotting top. Two years later, the aft pair of torpedo tubes were removed.
The ship received a final refit between 1934 and 1936, when her deck armour was increased to 5 inches (12.7 cm) over the magazines and to 3.5 inches (8.9 cm) over the engine rooms. In addition to a general modernisation of the ship’s systems, her anti-aircraft defences were strengthened by replacing the single mounts of the AA guns with twin mounts for the QF 4-inch Mark XVI gun and adding a pair of octuple mounts for two-pounder Mk VIII “pom-pom” guns to sponsons abreast the funnel.[ Two positions for “pom-pom” anti-aircraft directors were added on new platforms abreast and below the fire-control director in the spotting top. A HACS Mk III director replaced the Mk I in the spotting top and another replaced the torpedo director aft. A pair of quadruple mounts for Vickers .50 machine guns were added abreast the conning tower. The mainmast was reconstructed as a tripod to support the weight of a radio-direction finding office and a second High-Angle Control Station.] The forward pair of submerged torpedo tubes were removed and four experimental 21-inch torpedo tubes were added above water forward of ‘A’ turret.[