Obolensky was educated at Lynchmere Preparatory School, Eastbourne, and at the Lycée Pasteur in France before going up to Trinity College, Cambridge, where he distinguished himself with a Blue for lawn tennis.
Obolensky became a distinguished academic. He was a Fellow of Trinity College, Cambridge (1942-48, Honorary Fellow 1991-2001) and Lecturer in Slavonic Studies, University of Cambridge (1946-48). He became Reader in Russian and Balkan Medieval History, University of Oxford (1949-61) and subsequently Professor of Russian and Balkan History (1961-85, Emeritus 1985-2001). He was a Student, Christ Church, Oxford (1950-85, Emeritus 1985-2001). He was Vice-President of the Keston Institute, Oxford.
Obolensky's most enduring achievement was The Byzantine commonwealth (1971), a large-scale synthesis of the medieval history of eastern Europe. Other major studies include The Bogomils: a study in Balkan neo-Manichaeism (1946) and Six Byzantine Portraits (1988).
Obolensky was elected a Fellow of the British Academy (1974, Vice-President 1983-85) and appointed Knight Bachelor (1984). He was a member of the Athenaeum.
Obolensky married Elisabeth Lopukhin in 1947; the marriage was dissolved in 1989. He died without issue.