Founded in 1957 as a non-profit organisation under Sri Lankan law, the school was originally named The Colombo School for Overseas Children. It was located at Queen's Street in Colombo Fort when the school first opened on 23 September 1957. The founders were from diplomatic missions (British High Commission, Burmese High Commission and Dutch Embassy) and the business community; this still reflects the school population today.
In September 1961, the school leased the premises known as "Rippleworth" at Turret Road (Dharmapala Mawatha) in Colombo 3. The younger children remained at the Fort while the older children moved to Rippleworth. In August 1963 the school in Fort had 100 children, while the school at Rippleworth, known as the Upper School, had about 72 children. In 1967 the school gained accreditation by the International Schools Association in Geneva, Switzerland.
To cope with an increasing numbers of pupils, in 1971 the school relocated to 51 Muttiah Road in Colombo 2 (Rivington) until finally in 1983 the school was relocated to its present address Pelawatte, Battaramulla. During the same year the school obtained accreditation by the International Baccalaureate (IB) Organisation as an IB World School fully capable of offering the IB Diploma Programme. In 1982, the school changed its name to Overseas Children's School (OCS).
In 1990 the school received further accreditations from ECIS (European Council of International Schools) and MSA (Middle States Association of Colleges & Schools) and over the next five years opened its gymnasium and swimming pool. As the school developed it became recognised as an Office of Overseas sponsored school and has built close ties with the US Embassy while maintaining links with the other diplomatic missions. In 1996 the school took its present name, The Overseas School of Colombo. Continuing its campus development programme, the school built a new performing arts facility in 2005, renovated its primary and secondary libraries, and inaugurated a new, state-of-the-art sports facility in 2009.
The Overseas School of Colombo remains Sri Lanka's oldest internationally accredited educational institution. It is the only school in Sri Lanka to offer the International Baccalaureate Programme from Pre-school to Grade 12. OSC is a multinational English medium international school catering to the needs of the internationally mobile expatriate community and Sri Lankan students. At OSC we provide an educational setting that is focused on developing each student to achieve their full potential. That degree of personal excellence can only come with highly qualified teachers committed to the needs of all students on a campus which balances nature with state of the art technology, facilities and resources. OSC is a culturally diverse community with students, parents and staff working in harmony.