After 1919: At the Nietzsche-Archiv

In 1919, Oehler left the military with the rank of Major and went to work with Elisabeth Förster-Nietzsche at the Nietzsche-Archiv, beginning in April. He settled first in Bad Berka, and then in Weimar. He was very deferential towards his much older cousin and soon became her "right hand". His family in a way also became hers, as she had long lost her brother and husband and had never had children.
In addition to Max Oehler, Förster-Nietzsche's "Nietzsche-Archiv" also employed Richard Oehler (Max's brother) and Adalbert Oehler (a cousin). The Archiv operated like a family business. They had all shared an opposition to the democratic Weimar Republic and had been more or less sympathetic towards the Conservative Revolutionary movement and the rise of Fascism. Max Oehler was especially favorable towards Benito Mussolini, whom in 1925 he declared a true follower of Nietzsche. Nevertheless, Oehler and his wife were members of Hermann Graf Keyserling's anti-militarist "Schule der Weisheit".
In 1931, Oehler became a member of the Nazi party and hitched the Archiv's connections to the growing Nazi movement. Like Elisabeth Förster-Nietzsche, he embraced Adolf Hitler's rise to power. Hitler visited the Archiv on several occasions between 1932 and 1934.
After Förster-Nietzsche's death in 1935, Oehler became the de facto leader of the Archiv, which lost completely the character of a salon: Oehler instead guided visitors such as students, soldiers and guests through the Archiv and tried to popularize his Nazi view of Nietzsche in speeches and essays.
Following Elisabeth Förster-Nietzsche's death, it became known within the Archiv that she had forged some documents and anecdotes relating to her brother Friedrich; this fact was not made public, however. In fact, Oehler seems to have publicly renounced only one of Förster-Nietzsche's legends: in 1937, he showed in an article that the Nietzsche family did not descend from Polish nobility, as was claimed by both Elisabeth and Friedrich Nietzsche himself on occasion. Oehler's conclusion is accepted as correct today, and was very much approved by Nazi ideology.
The United States Army occupied Weimar in 1945, at the close of World War II, and later handed the city over to Soviet forces. Oehler attempted to defend the Nietzsche-Archiv from the charge of having supported the Nazi regime, claiming that it always had been politically neutral. He could maintain that soldiers did not use the Archiv rooms, but the Soviet administration nevertheless froze all the Archiv's accounts. On December 6, 1945, Oehler was picked up by someone claiming to be Soviet interpreter. He never returned. As the family later found out, he had been sentenced to penal labour in Siberia but had died, probably of hunger or freezing, in an improvised prison while still in Weimar.