Oehrn joined the Reichsmarine in 1927, serving aboard the light cruisersKönigsberg and Karlsruhe, before being one of the first officers to transfer to the newly formed U-boat arm in July 1935. He was appointed to command of U-14 in January 1936, and patrolled in Spanish waters during the Civil War in July–September 1936. In August 1939 he joined the staff of BdU as an Admiralstabsoffizier.[1]
In May 1940 Oehrn took command of U-37, in order to restore trust in the G7e/T2 torpedo, which had performed abysmally, often detonating prematurely or not at all. In four patrols he sank 23 merchant ships for a total of 103,821 GRT, damaged another of 9,494 GRT, and sank the British sloopHMS Penzance, before returning to the staff in August.[1]
From November 1941 Oehrn served on the Mediterranean U-boat staff, but during a mission to North Africa in July 1942, he was severely wounded and captured. After recovering at a British Military Hospital in Alexandria, Oehrn was sent to POW Camp 306 on the Suez Canal. He returned to Germany in October 1943 after being released in a prisoner exchange. Oehrn spent the remainder of the war serving on the staff in a number of posts.[1]
Busch, Rainer; Röll, Hans-Joachim (2003). Der U-Boot-Krieg 1939–1945 — Die Ritterkreuzträger der U-Boot-Waffe von September 1939 bis Mai 1945 [The U-Boat War 1939–1945 — The Knight's Cross Bearers of the U-Boat Force from September 1939 to May 1945] (in German). Hamburg, Berlin, Bonn Germany: Verlag E.S. Mittler & Sohn. ISBN978-3-8132-0515-2.
Fellgiebel, Walther-Peer (2000) [1986]. Die Träger des Ritterkreuzes des Eisernen Kreuzes 1939–1945 — Die Inhaber der höchsten Auszeichnung des Zweiten Weltkrieges aller Wehrmachtteile [The Bearers of the Knight's Cross of the Iron Cross 1939–1945 — The Owners of the Highest Award of the Second World War of all Wehrmacht Branches] (in German). Friedberg, Germany: Podzun-Pallas. ISBN978-3-7909-0284-6.
Scherzer, Veit (2007). Die Ritterkreuzträger 1939–1945 Die Inhaber des Ritterkreuzes des Eisernen Kreuzes 1939 von Heer, Luftwaffe, Kriegsmarine, Waffen-SS, Volkssturm sowie mit Deutschland verbündeter Streitkräfte nach den Unterlagen des Bundesarchives [The Knight's Cross Bearers 1939–1945 The Holders of the Knight's Cross of the Iron Cross 1939 by Army, Air Force, Navy, Waffen-SS, Volkssturm and Allied Forces with Germany According to the Documents of the Federal Archives] (in German). Jena, Germany: Scherzers Militaer-Verlag. ISBN978-3-938845-17-2.