Japanese cruiser Ōyodo

The ship's main battery comprised six 50-caliber 155 mm (6.1 in) 3rd Year Type guns in two triple-gun turrets superfiring forward of the superstructure.[8] The gun was originally developed as a dual-purpose (anti-surface and anti-aircraft) weapon for the Mogami class; when they had their triple 155 mm turrets replaced with twin 203 mm (8 in) turrets, the now-surplus turrets were mounted on Ōyodo (as well as the Yamato-class battleships). Their slow rate of fire of 5 rounds per minute and limited elevation (up to only 55 degrees) made them unsuitable for the anti-aircraft role. The guns fired 55.87-kilogram (123.2 lb) shells at a muzzle velocity of 950 meters per second (3,100 ft/s) to a maximum range of 27,400 meters (30,000 yd) at +45 degrees of elevation. Each gun was provided with 150 rounds.[9]

Ōyodo's heavy anti-aircraft battery consisted of eight 65-caliber 100 mm (3.9 in) Type 98 dual-purpose guns in four twin mounts.[8] The mounts had a range of elevation from -10 degrees to +90 degrees and the guns had a muzzle velocity of 1,010 m/s (3,300 ft/s) with their 13 kilograms (29 lb) projectiles. They had a range of 19,500 meters (21,300 yd) and a height of 13,000 meters (43,000 ft) at maximum elevation.[10] The anti-aircraft armament was completed by eighteen 60-caliber 25 mm (1.0 in) Type 96 light AA guns in six triple mounts.[8] Firing 0.25-kilogram (0.55 lb) shells at a muzzle velocity of 900 m/s (3,000 ft/s). These guns had an effective range of 1,500–3,000 meters (1,600–3,300 yd), and an effective ceiling of 5,500 meters (18,000 ft) at an elevation of 85 degrees. The maximum effective rate of fire was only between 110 and 120 rounds per minute because of the frequent need to change the fifteen-round magazines.[11]

The ship was equipped with a director-control tower above the bridge with a Type 94 gunnery director controlling the main armament. The director was fitted with a 6-meter (19 ft 8 in) rangefinder and the upper gun turret mounted a 8-meter (26 ft 3 in) rangefinder. The Type 98 10 cm guns were controlled by a pair of Type 94 directors positioned at the base of the bridge and the light AA guns were provided with three Type 95 directors, all on the forward superstructure.[8]

Like the Tone-class heavy cruisers, the Ōyodo-class ships were intended to be scouting cruisers and hence the entire deck of the ship aft of the superstructure was devoted to aircraft facilities. The weight saved by not fitting torpedo tubes was invested instead in a hangar that could house four floatplanes, with two more stowed on deck, and a heavy-duty 44-meter (144 ft) catapult that was necessary for the new Kawanishi E15K Shiun floatplane (Allied reporting name "Norm") that was intended to perform reconnaissance for the submarine flotilla in areas where the enemy had air superiority. To achieve this, the plane was designed with two retractable underwing stabilizing floats and a large central float that could be jettisoned to increase speed. However, the aircraft performed poorly and only fifteen were completed before it was canceled in February 1944. Ōyodo did carry two or three Aichi E13A "Jake" reconnaissance floatplanes during her career.[12]

The ship's armor was designed to protect against 155 mm shells and 250-kilogram (550 lb) bombs dropped from an altitude of 3,000 meters (9,800 ft). It had a waterline belt had a maximum thickness of 60 mm (2.4 in) of copper alloy homogeneous armor that protected the propulsion machinery and the bomb magazine; it extended 2.35 m (7 ft 9 in) above the waterline and 1.56 m (5 ft 1 in) below. The armored deck rested on the upper edge of the belt armor and was 30 mm (1.2 in) thick except over the bomb magazine where it increased to 50 mm (2.0 in). The ends of the belt armor were closed off by 35 mm (1.4 in) transverse bulkheads that also increased to 50 mm adjacent to the bomb magazine. The magazine itself had 35 mm sides and a 25 mm front. The steering gear aft was protected by an armored box with 40 mm (1.6 in) sides, a 20 mm (0.8 in) front and a 25 mm rear.[13]

Forward of the boiler rooms, the fire-control center, the 25 mm and 155 mm magazines had armored sides. The latter magazine had 75 mm (3.0 in) plates that tapered to 40 mm at their lower edge. The armor on the other compartments was 60 mm thick reducing to 30 mm at the bottom. The roof of these spaces was 50 mm thick over the 155 mm magazines and reduced to 28 mm over the other compartments. The main gun turrets were protected by 25 mm homogeneous armor plates and their barbettes had 20 mm or 25 mm thick plates. The sides of the conning tower were 40 mm thick and it had a roof 20 mm thick.[14]

Ōyodo in 1944

On 6 March, Ōyodo went into the dry dock at Yokosuka for a refit to convert her into a flagship, in which her hangar was converted into staff offices and accommodations to take advantage of her elaborate communications suite. The aircraft catapult was exchanged for the shorter (18 m (59 ft)) standard type, and a pair of Type 22 surface search radars were installed. 29 additional Type 96 guns were installed in six triple and eleven single mounts for a total of 47 weapons. The modifications increased her complement to 61 officers and 850 sailors. The day after the refit was finished on 31 March, Ōyodo became the flagship of the Combined Fleet. Admiral Soemu Toyoda, commander of the Combined Fleet, hoisted his flag on her on 4 May. Ōyodo remained in Japanese waters until 11 October. On 29 September, Admiral Toyoda and his staff transferred to the new underground headquarters of the IJN in Yokohama. Six additional Type 96 AA guns in single mounts were installed in early October, along with a Type 13 radar, and her Type 22 radars were modified to allow them to be used for fire control.[17]

Ōyodo rejoined the Main Body of the 1st Mobile Fleet, commanded by Ozawa, on 5 October and departed Yokosuka on 11 October. En route, she was attacked by the submarine USS Trepang which fired six torpedoes, all of which missed. The ship departed Yashima anchorage on 20 October 1944 towards the Philippines as part of Operation "Sho-Ichi-Go"— which was intended to defeat the American invasion of the Philippines. Ōyodo was part of Ozawa's Northern Mobile ("Decoy") Force, which was to bait the American aircraft carrier strike force away from the main Japanese strike force by exposing the surviving Japanese carriers. Ōyodo was the only warship in Ozawa's force that had reconnaissance floatplanes, and both E13A1's performed reconnaissance and anti-submarine patrols over the fleet.[15] On the morning of 24 October, the bulk of the few aircraft aboard the carriers were launched to attack the American carriers as a distraction. They inflicted no damage and caused the Americans to search in the direction from which they had attacked. The Americans finally spotted Ozawa's ships at 16:40, some 200 miles (320 km) east of Cape Engaño, the northeastern tip of Luzon. The American carriers were spread out and it was very late in the day to launch an airstrike, so Admiral William Halsey, commander of the Third Fleet decided to mass his carriers in a position to attack the following morning. Ozawa reversed course during the night, correctly believing that the Americans would follow him north.[18]

Although they had lost contact during the night, the Americans did find the Japanese carriers at 07:35. They had already launched an airstrike of 180 aircraft that was orbiting 50 miles (80 km) ahead of the American carriers while waiting for the Japanese ships to be located.[19] This was just the first of a total of five airstrikes that the Americans launched that day. During the first strike, Ōyodo suffered two near-misses, but at 08:48 she was hit by a bomb that damaged a boiler room. At 10:54, Vice Admiral Ozawa left the sinking aircraft carrier Zuikaku and transferred his flag to Ōyodo. Later in the day the ship was hit by two rockets from F6F Hellcat fighter-bombers and damaged by another near-miss and Ozawa ordered his remaining forces to retire northward. Around 19:00 Ozawa learned about a force of destroyers and cruisers that sank the light carrier Chiyoda and drove off the Japanese destroyers rescuing survivors from some of the carriers lost earlier in the day. He ordered the two Ise-class hybrid battleship/carriers and Ōyodo to reverse course and engage the Americans, but they were unable to find them, and they reversed course at 23:30 and headed for Amami Ōshima. When they arrived there on the 27th Ozawa transferred his flag to the battleship Hyūga[15]

Ōyodo sunk in shallow waters near Kure in 1945

A few days later Ōyodo was sent to Manila on a transport run, arriving on 1 November. Three days later she departed for Brunei and remained in the South China Sea area for the next several months. On 24 December Ōyodo, the heavy cruiser Ashigara and six destroyers sortied to attack the American forces at San Jose on the island of Mindoro. They were spotted by American aircraft late the next day and Ōyodo was hit by a pair of 500 lb (230 kg) bombs; one failed to detonate and the other only slightly damaged the cruiser. The destroyers engaged an American convoy escorted by PT boats while the cruisers fired star shells to illuminate the area. They later bombarded a supply dump and set it on fire before returning to Cam Ranh Bay, French Indochina.[15]

Ōyodo arrived in Singapore on 1 January 1945 and was repaired from 9 to 29 January. The following month she was assigned to Operation Kita together with the two Ise-class battleship/carriers and three destroyers. The ships were loaded with critically needed strategic war supplies (aviation gasoline, rubber, tin, zinc, and mercury) and 1,150 surplus oil workers to be ferried back to Japan. The ships sailed from Singapore on 10 February and was spotted by the British submarine HMS Tantalus the following day. Tantalus was forced to submerge by a maritime patrol aircraft and was unable to attack. On 13 February the submarine USS Bergall unsuccessfully attacked the ships as did the submarine USS Blower. Later that afternoon, Ōyodo launched one of her floatplanes which spotted the submarine USS Bashaw on the surface ahead of the convoy. Hyūga opened fire with her main guns and forced Bashaw to submerge when one of her shells landed within 1,600 meters (1 mi) of the submarine. The convoy reached the Matsu Islands, off the Chinese coast, on the 15th and was unsuccessfully attacked by the submarine USS Rasher before they reached Zhoushan Island, near Shanghai, that night. The convoy reached Kure on 20 February, having evaded or escaped pursuit by twenty-three Allied submarines along the way.[15]

Ōyodo was transferred to the Kure Training Force on 1 March, and, a few weeks later, Vice Admiral Marc Mitscher's Task Force 58 made the first carrier attack on Kure on 19 March. Three 500 lb bombs hit Ōyodo; one hit the port engine room and started fires that badly damaged the propulsion machinery compartments. Another hit the base of the funnel and destroyed the boiler uptakes and the last one detonated inside the ship. Near-misses ruptured some hull plating forward and the cruiser started to flood, but she was towed to Etajima and beached to prevent her from sinking. The bombs killed or wounded 54 crewmen. She was refloated and entered a dry dock for repairs on 23 March which were completed on 4 May.[20]

Ōyodo being scrapped in Kure, 27 April 1948

On 24 July US Task Force 38 launched a massive attack to destroy any and all remaining units of the Japanese Navy. Ōyodo was strafed and hit by five 250-pound (110 kg) bombs, two of which hit near her catapult and punched holes in her deck. Two more hit amidships near the engine rooms and the last struck forward of the bridge and started a fire that could not be extinguished for two days. Four days later, a day-long attack was launched by the US carrier fleet. Ōyodo was near-missed by bombs in the morning and the shock waves from their detonations ruptured her hull plating abreast of the forward engine room and No. 5 boiler room that flooded those two compartments. The asymmetric weight of the water on the starboard side caused her to capsize to starboard in shallow water about 25 minutes later. Permission to abandon ship was granted by Captain Shoichi Taguchi before she capsized, but about 300 crewmen were killed.[20]

Ōyodo was removed from the navy list on 20 November. Her wreck was raised on 20 September 1947 and towed to a dry dock to be demolished. The ship was scrapped from 6 January to 1 August 1948.[20]