At the end of the World War II, Oppermann decided on painting as his career choice at the age of fifteen and began writing at the same time.
After graduating from the Städtisches Fürst-Otto-Gymnasium, now the Gerhart-Hauptmann-Gymnasium Wernigerode, Oppermann left the GDR out of political conviction and studied at the Hochschule für bildende Künste in West Berlin from 1950.
In 1952, during a trip to Paris, he met Günter Grass, the later Nobel Prize for Literature winner, with whom he had been friends since their years of study together at the Berlin Academy of Fine Arts.
On the occasion of a first Latin America trip with exhibitions in Bogotá and Caracas, a newspaper review by Luis Freres referred to Oppermann as a "Prusiano-Latino" for the first time.
The term can be traced back to Oppermann's in-depth examination of the Prussian past, which resulted from his claim to political-historical enlightenment, as well as to his emotional attachment to Spain (German: "engagierter Preuße und Lateinamerikaner").
Numerous solo exhibitions subsequently took Oppermann to Caracas, Lima, New York City, São Paulo, and many European capitals such as Berlin, Brussels, Paris, London and Prague.
In 1995, Gebrüder Mann Verlag, Berlin, published a comprehensive biography of the painter under the title "Karl Oppermann – Prusiano-Latino".
Oppermann contributed paintings with international themes, which are now accessible to the general public as a donation or permanent loan in the Wernigerode University Library.