| Oblates of Mary Immaculate in Ceylon 1848-1948 | D.J.B. | Kuruppu | 1948 | Religion |
| Oblates Serve in Sri Lanka | W.L.A., Rev Fr | Don Peter | 2000 | Religion |
| Observer Annual 1950 | N/A | N/A | 1950 | General |
| Observer Annual 1992 | Rajpal | Abeynayake | 1992 | General |
| Observer Annual 1998 | Carol | Aloysius | 1998 | General |
| Observer Pictorial 1988 | Harold | Pieris | 1988 | General |
| Observer Pictorial 1991 | C.S. | Dharmarajah | 1991 | General |
| Octet - Collected Plays | Regi | Siriwardena | 1995 | Literature |
| October Revolution - The 50th Anniversary of the | Leslie | Goonewardene | 1967 | Politics |
| October, National Liberation and Ceylon | Pieter | Keuneman | 1967 | Politics |
| Odysey of a Runaway Child, Galle to Australia\'s Goldfields and Beyond | Manouri P. | Senanayake | 2018 | General |
| Odyssey & Living Legacy of Sieur de La Nerolle | Yasmin | Rajapakse | 2018 | History |
| Odyssey inti the Wild | B. A. | Mahipala | 2013 | General |
| Odyssey of the Tooth Relic, The - The Palladium of Sinhalese Royalty | H.S. | De Zylva | 1928 | Religion |
| Of Jasmines and Jamboos | Grace | Mackie | 1993 | General |
| Of Love and Squalor & Other Stories | Mark | Bartholomeusz | 1971 | Literature |
| Of Sri Lanka I Sing - The Life & Times of Devar Surya Sena | N/A | N/A | 1978 | General |
| Of Tamils & Tigers, a Journey through Sri Lanka's War Years | Ben | Bavinck | 2011 | General |
| Of The Father & Of The Son | James J. | Bulner | 1985 | Literature |
| Off Duty - The Ceylon Police Magazine, Vol XV, No 10, February 1941 | N/A | N/A | 1941 | General |
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