Armgard was born at Bad Driburg, Kingdom of Prussia (now in North Rhine-Westphalia, Germany), fourth child and daughter of Aschwin von Sierstorpff-Cramm (1846–1909), and his wife, Hedwig von Sierstorpff-Driburg (1848–1900). Armgard married on 24 October 1905 at Hanover to Count Bodo von Oeynhausen (1881–1909), an officer in the 8th Hussars in Paderborn, son of Count Erich von Oeynhausen and his wife, Therese von Lenthe.
Alexis accompanied Armgard to the wedding of Bernhard to Princess Juliana. During Armgard and Alexis were observed by the local Gestapo. Her apolitical past and the service that the monarchist and anti-Stalinist Colonel Pantchoulidzew later rendered in the war to the German Reich Railway, would have shielded her from Nazi authorities.
The Steamship demanded in September 1944 in Recke, one of Armgard"s properties, Schloss Woynowo Walde for military purposes.
Armgard and Alexis gave an account of the withdrawal in 1945 of the Wehrmacht behind the Oder-line on their estate at Neumark. Alexis went on to be the Netherlands" sole representative at the 1956 Summer Olympics, competing in dressage.
Armgard died in Diepenheim at the age of 87.
