| Open Letter to the Kandyan Chiefs, An | Ananda. K. | Coomaraswamy | 1957 | General |
| Open-Air Way of the Cross - Hiniduma, Ceylon | N/A | N/A | 1958 | Religion |
| Opening of the Indoor Cricket Nets - Souvenir | Sri Lanka Cricket Foundation | N/A | 1984 | General |
| Opera, UK - Vol 59, No 5, May 2008 (Danielle de Niese) | John | Allison | 2008 | General |
| Operation Elpitiya (JVP's Revolution in 1971) | Gunadasa | Liyanage | 1993 | Literature |
| Operational Aspects of The Change Agents Programme of Sri Lanka | A.B. | Talagune | 1985 | General |
| Ophidia Taprobanica or the Snakes of Ceylon | Frank | Wall | 1921 | General |
| Orang Rejimen - The Malays of the Ceylon Rifle Regiment | B.A. | Hussainmiya | 1990 | General |
| Oratorian Mission in Ceylon, The - Part 2 | S.G., Rev. Fr | Perera | 1936 | Religion |
| Oratorian Mission in Ceylon, The - Part 3 | S.G., Rev. Fr | Perera | 1936 | Religion |
| Oratorian Mission in Ceylon, The - Part 5 | S.G., Rev. Fr | Perera | 1937 | Religion |
| Oratorian Mission in Sri Lanka , The 1796-1874 | A. C. | Dep | 1987 | Religion |
| Orchids & Cacti | Reggie, Senator | Perera | 1968 | Literature |
| Orchids of Sri Lanka - A Simplified Guide to Identification, ICUN, Vol 1 | Malik | Fernando | 2003 | General |
| Organising For Development - Progress & Reaction in Ceylon | C.R. | Hensman | 1963 | General |
| Oriental Cookbook | Priya | Wickramasinghe | 1982 | General |
| Origin of Species According to the Buddha | Mahinda | Weerasinghe | 2002 | Religion |
| Origins of Sinhala Culture | S.D. | de Lanerolle | 1976 | General |
| Origins of the Ethnic Problem & Separatist Tamil (Eelam) Terrorsm in Sri Lanka | Tilak | Ratnakara | 1985 | General |
| Origins of the Pentecostal Mission in Sri Lanka | G. P. V. | Somaratne | 1996 | Religion |
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