In 1922, Oosterhuis went to Amsterdam to study medicine and after his study he became a doctor in Delfzijl. Thanks to his work as a cruiser, with his ships Cascade and Libelle, he was able to put up a smuggling route for the resistance between the harbour of Delfzijl and Stockholm. An important colleague of his in the Dutch resistance was the coaster-captain Harry Roossien who made many trips during the war. Due to these activities many people and materials left occupied Netherlands, and radio transmitters, photos from the Dutch Queen and money for the resistance were shipped into the country.
Oosterhuis was the leader of the resistance group 't Zwaantje (The Swan) from Delfzijl. The name comes from a pub named De witte Zwaan (The White Swan]) which Oosterhuis regular visited. He used the name Zwaantje as a codename in the illegal documentation he sent to the resistance and the allies.
On July 12, 1943, the German Sicherheitsdienst rolled up the resistance group after they were betrayed. Despite a collective death sentence on June 23, 1944, most of the members survived the war in captivity in German warcamps. They were liberated in the fall of 1945.
After the war, Oosterhuis, due to health reasons, quit his profession as a doctor and became a cruiser with his ship MS Stientje Mensinga, a rebuilt landing vehicle from 1943. The ship sunk during a heavy storm on the Irish coast by Erritshead in 1961.
In 1970, a resistance monument named 't Zwaantje' was revealed in Delfzijl.
- De verzetsgroep Zwaantje; oorlogsbelevenissen van dr. Allard Oosterhuis. Verteld door J.Klatter, uitgeverij N.V. Het Parool, Amsterdam 1968.