The submarine was laid down on 20 November 1937 as K XXI at the Koninklijke Maatschappij De Schelde, Vlissingen During construction she was renamed O 21, and was finally launched on 21 October 1939. Following the German invasion of 10 May 1940, the O 21 was hastily commissioned, still incomplete, and sailed for England on 12 May together with her sister and the tugboat B.V. 37, to be finally completed at the Navy yard in Rosyth.[1]
During the war she operated around England, the Mediterranean Sea, Colombo in the Indian Ocean and Fremantle off the west coast of Australia. She survived the war and was decommissioned on 2 November 1957 and sold for scrap the following year.[1]
Ships sunk and damaged by O 21.[2]
| Date | Ship name | Nationality/Type | Tonnage (GRT) | Fate |
|---|---|---|---|---|
| 28 July 1941 | Ernesto | Italian merchant | ? | missed |
| 3 August 1941 | ? | Italian barquentine | ? | Damaged |
| 5 September 1941 | Isarco | Italian merchant | 5738 | Sunk |
| 3 October 1941 | Oued Yquem | Vichy-French merchant | 1369 | Sunk |
| 22 November 1941 | San Salvatore | Italian schooner | 92 | Sunk |
| 28 November 1941 | German submarine | 769t ↑ 8710NaN0 ↓ | Sunk 36.4°N -23°W | |
| 13 March 1943 | Kasuga Maru No.2 | Japanese merchant cargo ship | 3967 | Sunk |
| 22 April 1943 | Yamazato Maru | Japanese army cargo ship | 6925 | Sunk |
| 29 July 1945 | ? | Japanese coaster | ? | Damaged |
| 29 July 1945 | ? | Japanese coaster | ? | Damaged |