| Oak Farm Craft Centre & Childrens Farm | Drayton Bassett | Staffordshire |
| Oak Farm Rare Breeds Park | Aylesbury | Buckinghamshire |
| Oak House | West Bromwich | West Midlands |
| Oakengates Theatre | Telford | Shropshire |
| Oaker Wood Leisure | Bucknell | Herefordshire |
| Oakham Castle | Oakham | Rutland |
| Oakham Tourist Information Centre | Oakham | Rutland |
| Oakhill Wood - London Wildlife Trust | Barnet | Hertfordshire |
| Oakhurst Cottage | Hambledon, Nr Godalming | Surrey |
| Oakleaf Brewing Company | Gosport | Hampshire |
| Oakwell Hall | Birstall | West Yorkshire |
| Oakwell Hall Country Park | Batley | West Yorkshire |
| Oakwood Leisure | Canaston Bridge, nr Narberth | Pembrokeshire |
| Oakwood Nurseries | Little Clacton | Essex |
| Oakwood Theme Park | Canaston Bridge, nr Narberth | Pembrokeshire |
| Oare Gunpowder Works | Faversham | Kent |
| Oasis | St Helens | Isle of Wight |
| Oasis Leisure Centre | Swindon | Wiltshire |
| Oathlaw Pottery & Gallery | Oathlaw | Angus |
| Oatlands Village | St Sampson | Guernsey |
| Oban Airport | North Connel, Oban | Argyll and Bute |
| Oban Art Gallery | Oban | Argyll and Bute |
| Oban Distillery Visitor Centre | Oban | Argyll and Bute |
| Oban Experience Visitor Centre | Oban | Argyll and Bute |
| Oban Highland Theatre Cinemas | Oban | Argyll and Bute |
| Oban Rare Breeds Farm Park | Oban | Argyll and Bute |
| Oban Sea Fishing | Oban | Argyll and Bute |
| Oban Sea Trek | Oban | Argyll and Bute |
| Oban Tourist Information Centre | Oban | Argyll and Bute |
| Oban War & Peace Museum | Oban | Argyll and Bute |
| Observatory Science Centre, The | Herstmonceux, Hailsham | East Sussex |
| Obsidian Art | Stoke Mandeville | Buckinghamshire |
| Occupation Tapestry Gallery | St Helier | Jersey |
| Ocean Beach Pleasure Park | South Shields | Tyne and Wear |
| Ocean Bowl | Falmouth | Cornwall |
| Ocean Commotion | Tenby | Pembrokeshire |
| Ocean Frontier | Fort William | Highland |
| Ocean Gallery | Swansea | Swansea |
| Ocean Lab | Fishguard | Pembrokeshire |
| Oceanarium | Bournemouth | Dorset |
| Oceanarium, The | St Davids | Pembrokeshire |
| Octagon Theatre | Bolton | Greater Manchester |
| Octagon, Orleans House Gallery, The | Richmond-upon-Thames | Greater London |
| Octavia Hill Birthplace Museum Trust | Wisbech | Cambridgeshire |
| Odda`s Chapel | Deerhurst | Gloucestershire |
| Odeon Cinema | Aylesbury | Buckinghamshire |
| Odiham Castle | Odiham | Hampshire |
| Odiham Waterlily Collection | Odiham | Hampshire |
| Offa`s Dyke | Nr Chepstow | Gloucestershire |
| Offshore Extreme | Newquay | Cornwall |
| Ogden Water Country Park | Ogden | West Yorkshire |
| Ogmore Castle | Ogmore | Bridgend |
| Okehampton Castle | Okehampton | Devon |
| Okehampton Tourist Information Centre | Okehampton | Devon |
| Old Barn Nurseries | Dial Post | West Sussex |
| Old Bayham Abbey | Lamberhurst | Kent |
| Old Beaupre Castle | Cowbridge | Vale of Glamorgan |
| Old Blockhouse | Old Grimsby | Isles of Scilly |
| Old Bloomsbury - The Literary London Pub Walk | Holborn | Greater London |
| Old Bridge House Museum | Dumfries | Dumfries and Galloway |
| Old Brook Pumping Station | Chatham | Kent |
| Old Bushmills Distillery | Bushmills | Co. Antrim |
| Old Byre Heritage Centre, The | Dervaig, Isle of Mull | Argyll and Bute |
| Old Camden Town Walk - Catacombs, Canals and Cafes | Camden | Greater London |
| Old Chapel Art Centre | Kirkwhelpington | Tyne and Wear |
| Old Chapel Gallery | Pembridge | Herefordshire |
| Old Chelsea Village Pub Walk | Sloane Square | Greater London |
| Old City Nobody Knows Walk | St Pauls | Greater London |
| Old Corn Mill and Pottery, The | Ilfracombe | Devon |
| Old Courthouse Museum | Knaresborough | North Yorkshire |
| Old Courthouse Museum, The | Beaumaris | Anglesey |
| Old Crown Court and Cells | Dorchester | Dorset |
| Old Dairy Farm Craft Centre | Weedon | Northamptonshire |
| Old Farmyard | Ripley, Nr. Harrogate | North Yorkshire |
| Old Fulling Mill Museum of Archaeology | Durham | Durham |
| Old Gala House & Christopher Boyd Gallery | Galashiels | Scottish Borders |
| Old Gaol Museum | Buckingham | Buckinghamshire |
| Old Gorhambury House | Nr St Albans | Hertfordshire |
| Old Grammar School | Castletown | Isle of Man |
| Old Guildhall Museum | East Looe | Cornwall |
| Old Haa Visitor Centre | Yell | Shetland |
| Old Hampstead Village London Walk | Hampstead | Greater London |
| Old House Museum, The | Bakewell | Derbyshire |
| Old House of Keys | Castletown | Isle of Man |
| Old House, The | Rochford | Essex |
| Old House, The | Hereford | Herefordshire |
| Old Jewish Quarter London Walk | Tower Hill | Greater London |
| Old Kensington - London's Royal Village | Kensington | Greater London |
| Old Knightsbridge Village Pub Walk | Kensington | Greater London |
| Old Lifeboat House | Portpatrick | Dumfries and Galloway |
| Old Lighthouse Dungeness | Dungeness | Kent |
| Old Man of Hoy | Hoy | Orkney |
| Old Manor, The | Norbury, Ashbourne | Derbyshire |
| Old Mayfair London Walk | Green Park | Greater London |
| Old Mill | Lower Slaughter | Gloucestershire |
| Old Mill Antique Centre | Bridgnorth | Shropshire |
| Old Mill Herbary | Helland Bridge, Bodmin | Cornwall |
| Old Mill, The | Wembury | Devon |
| Old Mills Watermills | Elgin | Highland |
| Old Operating Theatre, Museum & Herb Garret | Southwark | Greater London |
| Old Oswestry Hill Fort | Oswestry | Shropshire |
| Old Palace, The | Croydon | Greater London |
| Old Penny Memories | Bridlington | East Riding of Yorkshire |
| Old Police Station and Museum | Porthcawl | Bridgend |
| Old Royal Naval College | Greenwich | Greater London |
| Old Sarum Castle | Salisbury | Wiltshire |
| Old Smithy & Heritage Centre, The | Owston Ferry | North Lincolnshire |
| Old Smithy Craft Centre & Gallery, The | Haverfordwest | Pembrokeshire |
| Old Smithy Craft Shop and Heritage Centre | Newcastle Emlyn | Ceredigion |
| Old Smithy Tourist Centre | Godshill, Ventnor | Isle of Wight |
| Old Soar Manor | Plaxtol, Borough Green | Kent |
| Old Soho Pub Walk | Embankment | Greater London |
| Old Town Hall | Newtown | Isle of Wight |
| Old Town Hall Local History Museum | Margate | Kent |
| Old Trafford | Manchester | Greater Manchester |
| Old Wardour Castle | Tisbury, Salisbury | Wiltshire |
| Old Westminster By Gaslight | Westminster | Greater London |
| Old Westminster London Walk | Westminster | Greater London |
| Oldbury Court Estate | Bristol | Bristol |
| Oldham Coliseum Theatre | Oldham | Greater Manchester |
| Oldham Tourist Information Centre | Oldham | Greater Manchester |
| Oliver Cromwell's House | Ely | Cambridgeshire |
| Ollerton Mill | Ollerton, Newark | Nottinghamshire |
| Ollerton Tourist Information Centre | Ollerton, Newark | Nottinghamshire |
| Omagh Tourist Information Centre | Omagh | |
| On the Air | Chester | Cheshire |
| Once upon a Time | Woolacombe | Devon |
| Onchan Pleasure Park | Onchan | Isle of Man |
| Open Eye Gallery | Liverpool | Merseyside |
| Orbost Gallery | Dunvegan, Isle of Skye | Highland |
| Orchadian Stone Company | Golspie | Highland |
| Orchardton Tower | Palnackie | Dumfries and Galloway |
| Orchid Paradise | Newton Abbot | Devon |
| Ordsall Hall | Salford | Greater Manchester |
| Orford Castle | Orford | Suffolk |
| Orford Ness National Nature Reserve | Orford, Woodbridge | Suffolk |
| Oriel Albion Gallery | St Davids | Pembrokeshire |
| Oriel Emrys Gallery | Haverfordwest | Pembrokeshire |
| Oriel Glan y Mor Gallery | Fishguard | Pembrokeshire |
| Oriel Linda Norris Gallery | Clynderwen | Pembrokeshire |
| Oriel Mostyn Gallery | Llandudno | Conwy |
| Oriel Myrddin Gallery | Carmarthen | Carmarthenshire |
| Oriel Plas Glyn-y-Weddw Arts Centre | Llanbedrog | Gwynedd |
| Oriel Q - Queens Hall Gallery | Narberth | Pembrokeshire |
| Oriel Ynys Mon Rhosmeirch Museum & Gallery | Rhosmeirch | Anglesey |
| Oriel-Y-Felin Gallery & Tearoom | Trefin | Pembrokeshire |
| Original Ghost Walk of York | York | York |
| Original Great Maze, The | Braintree | Essex |
| Origins | Norwich | Norfolk |
| Orkney Fossil & Vintage Centre | Burray | Orkney |
| Orkney Marine Life Aquarium | Grimness | Orkney |
| Orkney Museum | Kirkwall | Orkney |
| Orkney Wireless Museum | Kirkwall | Orkney |
| Orleans House Gallery | Twickenham | Greater London |
| Ormesby Hall | Ormesby | Middlesbrough |
| Ormiston Market Cross | Ormiston | East Lothian |
| Ornamental Tree Nurseries Limited | Kingsland | Herefordshire |
| Ortak Visitor Centre | Kirkwall | Orkney |
| Orwell River Cruises | Great Blakenham | Suffolk |
| Osborne House | East Cowes | Isle of Wight |
| OSHY Gallery | Ambleside | Cumbria |
| Osprey Centre Loch Garten RSPB | Nethy Bridge | Highland |
| Osterley Park | Isleworth | Greater London |
| Ostrich Kingdom | Birniefield | Fife |
| Oswaldtwistle Mills Shopping Village | Oswaldtwistle | Lancashire |
| Oswestry Tourist Information Centre | Oswestry | Shropshire |
| Oswestry Transport Museum | Oswestry | Shropshire |
| Otley Hall | Otley | Suffolk |
| Otley Museum | Otley | West Yorkshire |
| Otter Trust's North Pennines Reserve | Bowes | Durham |
| Otter Trust, The | Bungay | Suffolk |
| Otterburn Mill | Otterburn | Northumberland |
| Otterton Mill | Otterton, Budleigh Salterton | Devon |
| Ottery St Mary Tourist Information Centre | Ottery St. Mary | Devon |
| Oulton Park Race Circuit | Little Budworth | Cheshire |
| Our Dynamic Earth | Edinburgh | Edinburgh |
| Outdoorlite Ventures in Wales | Llandovery | Carmarthenshire |
| Outwood Windmill | Redhill | Surrey |
| Over Bridge | Gloucester | Gloucestershire |
| Overbeck's Museum & Garden | Sharpitor, Salcombe | Devon |
| Owl & Bird of Prey Centre | Bridlington | East Riding of Yorkshire |
| Owl House Gardens | Tunbridge Wells | Kent |
| Owletts | Gravesend | Kent |
| Owlpen Manor | Dursley | Gloucestershire |
| Oxburgh Hall, Garden & Estate | Oxborough, King's Lynn | Norfolk |
| Oxford Airport | Kidlington | Oxfordshire |
| Oxford Story | Oxford | Oxfordshire |
| Oxford Tourist Information Centre | Oxford | Oxfordshire |
| Oxford University Museum of Natural History | Oxford | Oxfordshire |
| Oxfordshire Museum | Woodstock | Oxfordshire |
| Oxley Sports Centre | Sherborne | Dorset |
| Oxo Tower Wharf | South Bank | Greater London |
| Oxwich Castle | Oxwich | Swansea |
| Oxwich National Nature Reserve | Oxwich | Swansea |
| Oyster Brewery Bar and Restaurant | Easdale, Oban | Argyll and Bute |
| Oystermouth Castle | Mumbles | Swansea |
