In June 1933 Rothaug was named a prosecutor in Nuremberg, and in April 1937 he became the regional court director in Schweinfurt and director of Nazi "special courts" or "Sondergerichts" at Nuremberg. In 1938 he became a member of the German Nazi Party, though he had applied the year previously. He worked closely with the Sicherheitsdienst or intelligence apparatus of the Nazi SS.
Rothaug sought after and presided over the trial of Leo Katzenberger in March 1942, ordering his execution for "racial defilement" in May 1943. Rothaug accused the elderly Jewish man of having sexual relations with a younger German woman, Irene Seiler, which was a crime in Nazi Germany according to the Rassenschande or "racial purity" laws, a part of the Nuremberg Laws. Both Katzenberger and Seiler denied the accusations. Following the trial, Rothaug was brought to Berlin as a member of the Nazi People's Court.
During the Nuremberg Trials Rothaug was sentenced to life imprisonment on 14 December 1947 for crimes against humanity. He was the only defendant not to be convicted of all charges, being found guilty only of "crimes against humanity," and not guilty of "war crimes through the abuse of the judicial and penal process" and "membership in a criminal organization." Nonetheless, the court commented in its judgment that:
"By his manner and methods he made his court an instrumentality of terror and won the fear and hatred of the population. From the evidence of his closest associates as well as his victims, we find that Oswald Rothaug represented in Germany the personification of the secret Nazi intrigue and cruelty. He was and is a sadistic and evil man. Under any civilized judicial system he could have been impeached and removed from office or convicted of malfeasance in office on account of the scheming malevolence with which he administered injustice."
His sentence was later reduced to 20 years, and he was released on parole on 22 December 1956.
Rothaug's role in the Katzenburger trial was inspiration for the plot surrounding fictional character Ernst Janning in the 1961 film Judgment at Nuremberg.
- Ernst Klee: Das Personenlexikon zum Dritten Reich. Wer war was vor und nach 1945? S. Fischer, Frankfurt am Main 2003, ISBN 3-10-039309-0.
- Kohl, Christianne, "The Maiden and the Jew", Hoffman und Campe, Hamburg, 1997.
- Ernst Klee: Das Personenlexikon zum Dritten Reich. Wer war was vor und nach 1945. Fischer Taschenbuch Verlag, Zweite aktualisierte Auflage, Frankfurt am Main 2005, ISBN 978-3-596-16048-8, S. 510.
- Zitat bei Ernst Klee: Das Personenlexikon zum Dritten Reich, Fischer Taschenbuch 2005, S. 510.
- Kohl, Christianne, "The Maiden and the Jew" pp.140-148, Hoffman und Campe, Hamburg, 1997.