| 4301 | 2770 | Basil Oliver | The cottages of England : a review of their types and features from the 16th to the18th centuries | 1929 | B/22 | Oliver |
| 4302 | 3779 | Paul Oliver | The village green | ? | E/15 | UK/Oli |
| 4305 | 6814 | Paul Oliver | English cottages and small farmhouses : a study of vernacular shelter | 1975 | B/01.3 | Oliver |
| 4306 | 5191 | edited by R J Olney | Labouring life on the Lincolnshire Wolds : a study of Binbrook in the mid-nineteenth century | 1975 | E/17.1 | Olney, |
| 4307 | 5461 | O Olsen | Fyrkat, Viking Camp Near Hobro | 1959 | E/61.2 | Olsen, |
| 4311 | 3852 | The Countess of Onslow | Clandon Park | 1952/ | M/08 | Surrey |
| 4312 | 4368 | Peter Orde | Lindisfarne castle | ? | M/08 | Nation |
| 4313 | 3367 | Ordnance Survey | Map of southern Britain in the iron age | 1962 | E/61.5 | Ordnan |
| 4314 | 3354 | Ordnance Survey | Map of Britain in the dark ages from 410 AD to 870 AD | 1966 | E/61.7 | Ordnan |
| 4315 | 5045 | Bernard & Elizabeth Orna | Flint in Norfolk building | 1984 | B/05.4 | Orna, |
| 4316 | 4746 | edited by F J Osborn | Town & country planning | 1952 | E/16.7 | Osborn |
| 4317 | 4745 | edited by F J Osborn | Town & country planning | 1958 | E/16.7 | Osborn |
| 4318 | 50 | A L Osborne | A dictionary of English domestic architecture | 1954 | B/00.1 | Osborn |
| 4321 | 1455 | J F Oultram | The builder's foreman : a practical guide to his training | 1916 | B/02.3 | Oultra |
| 4322 | 4827 | ? | St. Mary the Virgin's church, Battle | ? | B/26.2 | Sussex |
| 4323 | 726 | L J Overton | Domestic hot water supplies & central heating by hot water | 1939 | WCP | Overto |
| 4324 | 1163 | Oxford Preservation Trust | Preservation of buildings of historical or architectural interest | 1966 | B/10.0 | Oxford |
| 4325 | 2243 | compiled by the Technical Committee of Oxford District Council of the Institute of Plumbing | Sheet roofing data book and design guide | 1982 | B/05.62 | Oxford |
| 4327 | 7190 | C Pace, J Birch | Look around the street | 1988 | E/16.6 | Pace, |
| 4328 | 7189 | C Pace, J Birch | Look around homes | 1988 | B/22 | Pace, |
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