In the spring of 1932, Leon Trotsky's son, Lev Sedov, who at that time was living in Berlin, was looking for a German-speaking secretary for his father.
Schüssler was offered the job and went to the island of Prinkipo in Turkey, where Trotsky was living in exile.
In 1933, Schüssler went back to Germany to attend some meetings of the German Trotskyist group, which had now been labeled an "illegal organization" after Adolf Hitler and the Nazi Party rose to power.
Otto Schüssler decided to move to Prague in Czechoslovakia, where he participated in the publication of illegal communist papers and pamphlets, using the pseudonym "Oskar Fischer."
When on May 24, 1940, a machine-gun assault by Mexican Stalinists took place against Trotsky's household, Otto Schüssler, together with Charles Cornell (one of Trotsky's guards from the United States and a member of the Socialist Workers Party), was arrested by the Mexican police.