O
| Ship | Country | Class and type | Displacement (tons) | First commissioned | Fate |
| Oakland | USA | Atlanta class light cruiser | 6,000 | 17 July 1943 | decommissioned 1949, scrapped 1959 |
| O'Brien | USA | Sims class destroyer | 1,570 | 2 March 1940 | sunk 19 October 1942 |
| Ocean | UK | Colossus class aircraft carrier | 13,200 | 8 August 1945 | scrapped May 1962 |
| Ōi | Japan | Kuma class light cruiser | 5,100 | 10 October 1921 | sunk by USS Flasher (SS-249) 19 July 1944 |
| Okinoshima | Japan | minelayer | 4,290 | 30 September 1936 | sunk by USS S-42 12 May 1942 |
| Oklahoma | USA | Nevada class battleship | 29,000 | 2 May 1916 | sunk at Pearl Harbor 7 December 1941; raised in 1943 and stricken in 1944; sank under tow in Pacific 1947 |
| Oklahoma City | USA | Cleveland class light cruiser | 11,800 | 22 December 1944 | converted to missile cruiser 1960, sunk as target 1999 |
| Oktyabrskaya Revolutsiya (originally Gangut) | Soviet Union | Gangut class battleship | 23,000 | 3 June 1909 | scrapped 1959 |
| Olav Tryggvason, later Brummer | Norway, then Germany |
minelayer | 1,924 | 21 June 1934, in German service from 9 April 1940 | Captured by the Germans 9 April 1940, renamed Brummer, sunk May 1945 |
| Oldenburg | Germany | minelayer | 1,141 | captured 13 September 1943 (former Italian water tanker Garigiliano, first renamed Dwarsläufer) | returned to merchant service 1945 |
| Omaha | USA | Omaha class light cruiser | 7,050 | 24 February 1923 | Scrapped Feb 1946 |
| Ordronaux | USA | Benson class destroyer | 1,620 | 13 February 1943 | decommissioned 1947, scrapped 1973 |
| Orella | Chile | Serrano class destroyer | 1,090 | ||
| Orion | UK | Leander class light cruiser | 7,000 | 18 January 1934 | scrapped 1949 |
| Orion | Germany | auxiliary cruiser | 15,700 | 9 December 1939 (former freighter Kurmark) | sunk 4 May 1945 |
| Orkan | Poland | M class destroyer | 1,920 | 18 November 1942 (former HMS Myrmidon | sunk 8 October 1943 |
| Orzeł | Poland | Orzeł class submarine | 1,100 | 2 February 1939 | sunk 11 June 1940 |
| Ouragan | France Free French, Poland & UK |
Borrasque class destroyer | 1,319 | 19 January 1927 | Polish Navy 17 July 1940; transferred to Free French 30 April 1941 then to Royal Navy in 1943; scrapped 7 April 1949. |
| Ōyodo | Japan | Oyodo class light cruiser | 11,433 | 28 February 1943 | sunk 25 July 1945 |
Read more about this topic: List Of World War II Ships