Oster was born in 1887 in the city of Dresden. He was a pastor’s son and joined the German military at the age of 20 in 1907. In the First World War he served with distinction on the Western Front. Indeed, in 1916 he was promoted to a captain in the German General Staff. Then, after Germany’s defeat, he was one of a few officers allowed to keep their jobs within the reduced army.
In 1932, however, Oster’s personality caught up with him. He ended up embroiled in a scandal involving an extra-marital affair. By the end of the year he was forced to resign his post. In a way though, this change of circumstances set the scene for Oster’s actions during the Second World War.
To begin with, like many Germans, Oster was supportive of the Nazi Party’s rise to power. But that all changed in 1934. By that time, Oster had secured a position in the Abwehr. It was here that Oster’s feelings about the regime began to dramatically change.