| On the Road Over the Ferries | Gnanamuttu | Times of Ceylon Annual | 1962 | N/A | N/A | N/A | General |
| On the Territorial Bahaviour of the Agamid Lizard | Kratzer | Loris | 1980 | XV | 3 | 162 - 163 | General |
| On Trees & Forests | Stevens | Loris | 1994 | XX | 3 | 131 - 133 | General |
| One Hundred & Eighty Miles on the Mahaweli Ganga | Markley | Loris | 1970 | XII | 1 | 46 - 52 | General |
| One Large Nature Reserve or Several Small Ones? Applying the Island Biogeographic Theory to Sri Lanka | Wickramaratne | Loris | 1988 | XVIII | 2 | 50 - 53 | General |
| Open Economy & Its Impact on Ethnic Relations in Sri Lanka | Gunasinghe | Sri Lanka - The Ethnic Conflict | 1984 | N/A | N/A | 197 - 214 | General |
| Open Prison Camp at Pallakelle, The | Pillai | Ceylon Today | 1960 | 9 | 6 | 6 - 11 | General |
| Open Prison Camp in Pallakelle, The | N/A | Ceylon Today | 1957 | VI | 1 | 19 - 20 | General |
| Operation Monkey | Somanader | Ceylon Today | 1966 | 15 | 10 | 12 - 17 | General |
| Operational Research in the Health Services | Jayasinghe | Ceylon Today | 1964 | 12 | 6 | 10 - 19 | General |
| Orange Headed Ground Thrush, An | Jinasena | Loris | 1988 | XVIII | 1 | 13 | General |
| Orange Orchards in the Gal Oya Valley | Punchihewa | Ceylon Today | 1969 | XVIII | 3 & 4 | 42 - 45 | General |
| Orchids of Ceylon, The | Soysa | Ceylon Today | 1954 | 3 | 3 & 4 | 9 - 15 | General |
| Orchids of Ceylon, The | Kriekenbeek | Tourists' Ceylon | 1968 | 2 | 1 | 40 - 43 | General |
| Oriental & Occidental Art | Howes | Ceylon Today | 1952 | 1 | 4 | 8 - 12 | General |
| Orientation in Migrating Birds - A Mystery Remains | Samayawardhena | Loris | 1999 | 22 | 1 | 33 - 34 | General |
| Origin & Development of Simhala Nadagam, The | Peiris | Journal of the Royal Asiatic Society | 1974 | XVIII New Series | N/A | 27 - 40 | General |
| Origin & Early Development of Grammatical Studies in Sinhala, The | Jayawardhana | Journal of the Royal Asiatic Society | 1989/90 | XXXIV New Series | N/A | 65 - 77 | General |
| Origin of the Kandyan Realm, The | Nell | Journal of the D.B.U. | 1962 | LII | 3 & 4 | 3 - 7 | History |
| Origin of the Muturajawela Canal | Somaratne | University of Colombo Review | 1984 | 1 | 4 | 94 - 98 | General |
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