In 1938, when Austria was annexed to the Third Reich, Oppenheim was expelled as a Jew from the school in which he taught for decades, thus he and his wife intended to emigrate to Australia where their daughters lived, but because of his illness they were not able take the trip. In 1942, he and his wife Amalie were transported to the concentration camp in Terezín where he died of diabetes in 1943, in that he was not given the necessary insulin-based medication. After World War II, his widow was able to move to Melbourne with a part of his archives.