| Once Upon An Island | Premnath | Moraes | 1993 | General |
| One & Many - An Inquiry Into The Problem of Plurality | J.F. | Samaranayake | 1978 | General |
| One Country One People | Sharadha | de Saram | 1993 | Literature |
| One Distinguished Family of Moor Street Jaffna | Marina | Ismail | 2014 | General |
| One Fourteenth of an Elephant, A Memoir of Life & Death on the Burma-Thailand Railway | Ian Denys | Peek | 2013 | General |
| One Hundred & Twenty Five (125) Years of Service | Ceylon Chamber of Commerce | N/A | 1964 | General |
| One Mad Bid For Freedom | James | Goonewardene | 1990 | Literature |
| One More Sunrise | Maurice | Perera | 2003 | Literature |
| One-Hundred-Year-Old Aviator recounts Pioneering Years of Civil Aviation in Sri Lanka | D.L. | Sirimanne | 2021 | General |
| Only in Sri Lanka | Gloria | Meltzer | 2015 | Literature |
| Only Man is Vile - The Tragedy of Sri Lanka | William | McGowan | 1992 | General |
| 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 |
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