On 25 April 1941, during a mission to place Special Operations Executive (SOE) agents in Collioure, on Roussillon coast in southern France, Guérisse and three crewmen from HMS "Fidelity" were arrested by the Vichy French coast guard and taken to a camp for British military prisoners at Saint-Hippolyte-du-Fort near Nîmes. Helped by 'fellow British' officers, O'Leary escaped in early June 1941. He went to Marseilles where everyone at the Fort knew there was an escape organisation run by a man named Ian Garrow and soon made contact. At this point his intention was make his way to Gibraltar and resume his original naval service. Events were to dictate otherwise because Garrow wanted O'Leary to stay and help with the organisation since he was a military officer, he had undercover training and unlike Garrow, spoke French fluently. Consequently, a request was sent to London that he stay, which was approved and confirmed by a BBC radio message received on 2 July 1941. "Fidelity" was lost with all hands when torpedoed in the South Atlantic on 31 December 1942. O'Leary immediately began his job : within a four-month period, he helped about fifty men escape from the prison of St Hippolyte du Fort, then moved them down the line back to England through the Pyrenees.