After the war O’Brien continued to live in Spain and perform services for the Spanish government. In 1940 she wrote "The Beseiged of the Alcazer" about a devout Catholic soldier fighting in the Siege of the Alcázar. She is credited as a writer in 1943 comedy film "Castillo de naipes". In 1950 she married Felix Baron von Vittinghof gennant Schell zu Schellenburg () in Madrid. The couple later relocated to Castle Kalbeck in Weeze in Germany, the baron's ancestral home. O’Brien died in Germany in 2000, aged 87.