| 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 |
| Oru & The Yatra, The - Traditional Out-Rigger Water Craft | V. | Vitharana | 1992 | General |
| Our Colony 1880 - Handbook of Victoria, Pictorial & Descriptive | James, Rev | Ballantyne | 1880 | General |
| Our Economic Needs | M.S., Mudaliyar | Karapper | 1940 | General |
| Our George - A Biography of George Edmund De Silva | Jane | Russell | 1981 | Politics |
| Our Heritage - Book 1 (Up To 1500) | G.C. | Mendis | 1969 | History |
| Our Heritage - Book 2 (1500 - 1796) | G.C. | Mendis | 1962 | History |
| Our Heritage - Book 3 (1796 - Second World War) | G.C. | Mendis | 1962 | History |
| Our Lady of Lanka | N/A | N/A | N/A | Religion |
| Our Lady of Lanka | John M | Senaveratna | 1945 | Religion |
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