Upon graduation in 1905, he became a French language and literature teacher in Giurgiu. In 1907, he transferred to Traian High School in Turnu Severin, remaining there until 1920, including a stint as principal. Among his pupils was the future literary critic Șerban Cioculescu, who recalled him as a stern disciplinarian. During summer holidays, alone or with students and teachers, he would travel to art museums in Austria, Germany, Italy and France. He declared his support for the 1907 Romanian Peasants' Revolt, and in January 1917, during World War I, after he had condemned the German occupation authorities, he was among sixty intellectuals and bureaucrats who were arrested in Turnu Severin. Afterwards, Oprescu was sent to Bulgaria, where he was interned in a camp before being freed several months later. Subsequently, he became an associate professor at the University of Cluj, which was located in a region that had come under Romanian jurisdiction with the union of Transylvania with Romania. At Cluj, he continued to teach French language and literature, also establishing and leading an art history seminar. While attending specialized courses in France, he became close friends with Henri Focillon, whose biography he later wrote, and whose letters to Oprescu were published posthumously.