Light Cruiser Ōi (Torpedo Cruiser)

Ōi (大井) was the fourth of five Kuma-class light cruisers, laid down on 24 November 1919 at Kawasaki Shipyard in Kobe, launched on 15 July 1920, and commissioned on 3 October 1921. Named after the Ōi River in Shizuoka Prefecture.

Like her sister ship Kitakami, Ōi was converted in 1941 into a torpedo cruiser (重雷装巡洋艦) equipped with ten quadruple 61 cm torpedo launchers for a total of 40 tubes. The pair represented the Imperial Japanese Navy's ultimate expression of torpedo warfare doctrine — the idea that a massive simultaneous torpedo salvo could decide a fleet engagement. In KanColle, Ōi is depicted as deeply devoted to Kitakami, reflecting their shared history as the Navy's only torpedo cruiser pair.

The torpedo cruiser conversion proved tactically unworkable, and in 1942 the torpedo armament was cut back to 24 tubes. Ōi was converted to a fast transport, carrying troops and supplies to forward areas in Southeast Asia and the Pacific.

On 19 July 1944, while steaming through the South China Sea with the destroyer Shikinami, about 570 miles south of Hong Kong, Ōi was intercepted by the American submarine USS Flasher (SS-249), commanded by Commander Reuben Whitaker. Flasher fired four stern torpedoes; two hit, one of them a dud, flooding the aft engine room, and Ōi sank about six hours later. Shikinami rescued her captain and 368 men; 153 were lost. USS Flasher would go on to become one of the highest-scoring American submarines of the war by tonnage sunk.