Onciul studied at Chernivtsi University, where he was active in Arboroasa and then in Societatea Academică Junimea, and the University of Vienna. In 1884, he received his doctorate in philosophy from Chernivtsi University. He became a professor at the University of Bucharest and director of the National Archives of Romania. He was the first chairman of the Advisory Heraldic Commission. Together with Ioan Bogdan, Onciul founded a school-of-thought in Romanian historiography that approached history critically. He dealt with the issue of Romanian origin, demonstrating the formation of the Romanian people over a wide area on both sides of the Danube and rejected the theory of medieval migration of Romanians from the Balkan Peninsula. Much of his work dealt with and documented the formation of the early Romanian feudal states. He sought to separate Medieval realities from 20th century politics.
- Teoria lui Roesler. Studii asupra stăruinții românilor în Dacia Traiană (Roesler's theory: Studies on the Persistence of Romanians in Dacia Felix) (1885)
- Radu Negru și originile Principatului Țării Românești (Radu Negru and Origins of the Principality of Wallachia) (1890-1892)
- Originile Principatelor Române (The Origins of the Romanian Principalities) (1899)
- Ideea latinității și a unității naționale (The Concept of Rome and National Unity) (1919)
- Tradiția istorică în chestiunea originilor române (Historical Tradition in the Issue of Romanian Origin) (1906-1907)
The Romanian Academy established an historiography prize in his name, known as the "Dimitrie Onciul Award"
The street, Strada Dimitrie Onciul, in Bucharest is named after him.
- Boia, Lucian (2001). History and Myth in Romanian Consciousness. Budapest: Central European University Press. p. 57. ISBN 963-9116-97-1.
- Anghelescu, Gheorghe F. (2011). Dimitrie Cantemir Savant de Dimensiune Europeană: 300 de Ani de la Plecarea din Domnie. Bucharest: Mica Valahic. p. page 85, note 86.
- Academia Republicii Populare Române - Dicționar Enciclopedic Român (Romanian Encyclopedic Dictionary) (ed. Bucharest, 1962-1964)
| President of the Romanian Academy
Iacob C. Negruzzi