| OH Honey Bees Ltd | Broxbourne | 2023 |
| Oherc Ltd | St Ola | 2015 |
| Ohoka Organic Limited | Tarporley | 2026 |
| Oilquick Investments Ltd | Sidcup | 2013 |
| Oink Events Limited | Cirencester | 2022 |
| OJ Polo Ponies Limited | Cheltenham | 2021 |
| OJM Tree & Landscape Services Ltd | Durham | 2009 |
| OJR Services Ltd | Farnham | 2024 |
| OK Equine Ltd | Leeds | 2022 |
| OK Farms Limited | Great Missenden | 2011 |
| Okabe Leisure Limited | Farnborough | 2020 |
| Okeford Farm Limited | Weymouth | 2025 |
| Okeford View Limited | Sturminster Newton | 2014 |
| Okehelm Limited | Dereham | 1989 |
| Okewood Limited | Blandford Forum | 2022 |
| OKO Oluto (UK) Limited | Canterbury | 2020 |
| Okpu and Family Farms Ltd | Edinburgh | 2021 |
| Olabbafrican Limited | London | 2025 |
| Olbek Fishing Company Limited | Grimsby | 1967 |
| Old Bells Farm Ltd | Stowmarket | 2025 |
| Old Berkshire Hunt Limited | Cirencester | 2004 |
| Old Buckenham Stud Limited | Newmarket | 1994 |
| Old Byland Woodlands Limited | Driffield | 2003 |
| Old Castle Wood Limited | Aberfeldy | 2004 |
| Old Chads Orchard Ltd | Malpas | 2014 |
| Old Chapel Co-Operative Ltd | Caersws | 2002 |
| Old Childsbridge Lane Woodland Limited | Sevenoaks | 2011 |
| Old Cornmill Staveley Paddock Limited | York | 2009 |
| Old Downs Farm Limited | Faversham | 2011 |
| Old Fargie Potato Breeding Co Limited | Glenfarg | 2012 |
| Old Farm Dogs Limited | Milton Keynes | 2025 |
| Old Forge Apiaries Ltd | Cambridge | 2018 |
| Old Glossop Anglers Community Interest Company | Glossop | 2026 |
| Old Granary Stables Limited | Ingatestone | 2023 |
| Old Green Stud Limited | Edinburgh | 2017 |
| Old Hall Farm Company Limited | Kings Lynn | 1949 |
| Old Hall Farm Winmarleigh Ltd | Preston | 2021 |
| Old Hanging Wood CIC | Ashford | 2020 |
| Old Hendre Farm Services Limited | Monmouthshire | 1996 |
| Old House Farm Equestrian Ltd | Woodhall Spa | 2026 |
| Old Kiln Fisheries Limited | Wirral | 2021 |
| Old Manor Farm Ltd | Henlow | 2017 |
| Old Manor Partnership Limited | Horsham | 2020 |
| Old Milestone Nursery Limited | Trowbridge | 2004 |
| Old Mill Dairies Limited | Carrickfergus | 2024 |
| Old Mill Equestrian Centre Ltd | Barnet | 2002 |
| Old Northwick Stables Limited | Stourbridge | 2009 |
| Old Oak Forest, Garden & Home Services and Trading Ltd | Taunton | 2021 |
| Old Oast Farms Ltd | London, | 2026 |
| Old Orchil Manor Ltd | Dunblane | 2019 |
| Old Park Farm (Bosham) Limited | Chichester | 2004 |
| Old Park Farm (Toddington) Limited | Northampton | 2016 |
| Old Park Farm Limited | Devizes | 1964 |
| Old Parkside Farm Ltd | Lancaster | 2023 |
| Old Plaw Hatch Farm Limited | East Grinstead | 1980 |
| Old Potash Farm Limited | Southam | 2015 |
| Old Rectory Farm Enterprises Limited | Droitwich | 2017 |
| Old Road Securities Limited | Bedford | 1973 |
| Old Rock Equestrian Limited | Cheltenham | 2021 |
| Old Silloth Farm Limited | Cambridge | 2020 |
| Old Sodbury Tree and Plant Nursery Ltd | Badminton | 2022 |
| Old Station Kennels Limited | Melton Mowbray | 2024 |
| Old Stour Broilers Limited | Salisbury | 2018 |
| Old Stour Poultry Limited | Salisbury | 2015 |
| Old Toll Nursery Ltd | Newtown | 2018 |
| Old Town Cove Ltd | St. Austell | 2025 |
| Old Walls Vineyards Ltd | Brixham | 2002 |
| Oldbury Cottage Care Farm CIC | Nuneaton | 2019 |
| Oldbury Cottage Farm Ltd | Nuneaton | 2016 |
| Oldbury Farm (Great Wakering) Ltd | Chelmsford | 2016 |
| Oldefield Equestrian Centre Limited | Beaconsfield | 2003 |
| Oldenburg Training Center Ltd | Lewes | 2023 |
| Oldershaws of Moulton Limited | Spalding | 1984 |
| Oldfield Construction Limited | Llanfairpwllgwyngyll | 2002 |
| Oldford Farms Limited | Carlisle | 2014 |
| Oldford Texels Limited | Mold | 2013 |
| Oldsport Outfitters Ltd | Leighton Buzzard | 2020 |
| Oldtown Farm Limited | Aberdeen | 2026 |
| Oldwood Pits Ltd | Bath | 2025 |
| Olgrinmore Forest Ltd | Thurso | 2011 |
| Olifant Organics Limited | Welwyn Garden City | 2024 |
| Olisah Limited | Glasgow | 2024 |
| Olivants Ltd | Midhurst | 2018 |
| Olive A.C Ltd | Cardiff | 2014 |
| Olive Farm (Babcary) Limited | Yeovil | 1977 |
| Olive Grove Salads Limited | Burscough | 2004 |
| Oliver Bros (Farming) Limited | Bridlington | 2015 |
| Oliver Brown Limited | Mansfield | 2008 |
| Oliver Copse Limited | Reading | 2010 |
| Oliver Cutts Agriculture Ltd | Fordingbridge | 2015 |
| Oliver Davies Sporting Ltd | Bridgnorth | 2018 |
| Oliver Forestry Limited | Carlisle | 2024 |
| Oliver Nutland Contracting Limited | Taunton | 2003 |
| Olivers Enterprises Limited | Earls Colne | 2012 |
| Olivers Equestrian Limited | St. Albans | 2022 |
| Olivers Tree Services Limited | Ferryhill | 2002 |
| Olivia Farming Holdings Limited | Alton | 2005 |
| Olivia Fishing Ltd | Lairg | 2022 |
| Olivia Jean Ltd | Annan | 2007 |
| Olivia Langmead Farming Limited | Alton | 2005 |