In 1920, Margarethe began work as a secretary in order to support her family financially. In 1925, she took a position as a secretary in the
Defense Ministry and in 1928 she was sent to the
military attaché's office in
Moscow for six months, under an assumed name. She was on a secret mission at the time when the
Reichswehr and
Red Army were still collaborating. From 1930 to 1935 she worked in Berlin at the
Reichswehr Ministry. In 1938 she moved to
Budapest and in 1940 to
Lisbon, as a secretary in the German military attaché office.
At the Bendlerblock In her prewar Berlin days, Oven had worked as a secretary for Colonel General
Kurt von Hammerstein-Equord and Colonel General
Werner von Fritsch in the
Bendlerblock headquarters of the Defense Ministry. In the summer of 1943, Major General
Henning von Tresckow requested that she be transferred to his office, placing complete trust in her because she was his wife's best friend since childhood. Through her relationship with Tresckow, Oven was drawn into the preparations for the 20 July 1944 coup attempt against Hitler known as the
20 July Plot or the
Valkyrie Conspiracy. When Tresckow was posted to the
Eastern Front, Margarethe was reassigned as a secretary in the headquarters of
Army Group Centre, serving as a news headline monitor for the Berlin coup plotters. She also typed the orders and decrees in preparation for the coup. During the summer and fall of 1943 she frequently met with Tresckow and
Claus von Stauffenberg outside the Bendlerblock, in order to discuss and modify orders, and was fully aware of the plot.
Arrest After the failure of the assassination attempt on 20 July 1944, Oven was arrested and held for two weeks, but after questioning she was allowed to return to her job at the Bendlerblock. ==Post-war==