Originally from Sri Lanka, he finished his studies in France. In 1983 he joined a team of the CNRS at the École Normale Supérieure to further his studies. In 1989, he became Research Specialist (Chargé de Recherche) at CNRS.
In 1991, Osmund Bopearachchi published an extensive work on Indo-Greek and Greco-Bactrian coinage, Monnaies gréco-bactriennes et indo-grecques. Catalogue raisonné (400 pages), which became the reference in its field. His conclusions are based on extensive numismatic analysis (find places, overstrikes, monograms, metallurgy, styles), classical writings, and Indian writings and epigraphic evidence. The book received the Prix Mendel (Mendel prize) for Historical Research in 1992.