| Ollands Farming Limited | Bury St. Edmunds | 2022 |
| Ollie Morton Tree Care Ltd | Hexham | 2019 |
| Ollie Noscoe Arborist Ltd | Poole | 2022 |
| Ollies Pest Control Hampshire Ltd | Aldershot | 2022 |
| Olly Murphy Racing Ltd | Stratford-Upon-Avon | 2016 |
| Olmo Land Management Ltd | Wells | 2020 |
| Olnafirth Sea Farm Limited | Shetland | 1988 |
| Olney Bloodstock Limited | St Ives | 2014 |
| Olsten Trout Limited | Louth | 2004 |
| OLY RAY Limited | Lincolnshire | 2023 |
| Olympia Hatcheries Limited | Nottingham | 2024 |
| Olympus Saddlery Ltd | Dormansland | 2013 |
| OM Holidays Ltd | Weymouth | 1958 |
| OM Penmon Limited | Manchester | 2012 |
| Omallys Ltd | Fraserburgh | 2016 |
| Omar Aziz Ltd | Woodbridge | 2018 |
| Omega Farms Limited | Cardiff | 2018 |
| OMG Hydroponics Ltd | Whitley Bay | 2014 |
| Omnidogs Limited | Huntingdon | 2021 |
| Omnis Equus Ltd | Guildford | 2022 |
| Omniserve Ltd | Wotton-Under-Edge | 2022 |
| Omoboma Limited | London | 2018 |
| OMP Stud Limited | Farnham | 2011 |
| Omutumba Investment Ltd | Southampton | 2023 |
| ON Guard Fencing Services Limited | Norwich | 2024 |
| ON Tiree Ltd | Coventry | 2018 |
| Onal Ltd | London | 2015 |
| Once Upon a Rescue Ltd | Watford | 2021 |
| Onco Trade Limited | London | 2022 |
| Oncoland Limited | Longfield | 1965 |
| Ondar Fishing Company Limited | Dorking | 1981 |
| Ondie Limited | Bromsgrove | 2017 |
| One 2 Tree Surgery Ltd | Carlisle | 2016 |
| One and Done (Carp Fishing) Ltd | Waterlooville | 2026 |
| One Big Drone Limited | Hullbridge | 2023 |
| One EAR Consulting Ltd | Larne | 2024 |
| One Garden Brighton Limited | Plumpton | 2019 |
| One Life Bloodstock Limited | Salisbury | 2021 |
| One Life Panama Limited | London | 2024 |
| One Loft Race Ltd | Cheltenham | 2025 |
| One Loft Specialist Racing Limited | Leeds | 2024 |
| One MAN and HIS LOG Ltd | Colchester | 2013 |
| One Percent 4 Art Community Interest Company | London | 2013 |
| One Plaice Limited | Aberdeen | 2014 |
| One Project Macclesfield Community Interest Company | Macclesfield | 2021 |
| One Stop Limited | Inverness | 1993 |
| One Tree Farm Ltd | North Curry | 2023 |
| One Two Tree Contracting Limited | Eastleigh | 2025 |
| One World Shipping Limited.Lkr Ltd | London | 2025 |
| One-Iron Limited | Leighton Buzzard | 1985 |
| Onechess Limited | Orpington | 2015 |
| Onefarm Limited | London | 2019 |
| Onepotplant Company Ltd | Sunderland | 2021 |
| Onfarmhatching Ltd | Northallerton | 2020 |
| Ongar Farms Limited | Coggeshall | 1956 |
| Ongo BIO Ltd | Middlesbrough | 2020 |
| Onley Farms Limited | Dunchurch | 1996 |
| Only Farms Ltd | Linwood | 2025 |
| Only Fish Ltd | Hailsham | 2023 |
| Only KOI Ltd | Cambridge | 2026 |
| Onlypaws Grooming Leeds Limited | Leeds | 2024 |
| Ontology Labs Ltd | Cambridgeshire | 2016 |
| Ontrak Nutrition Limited | Tarporley | 2005 |
| Ontrak Solar Limited | South Molton | 2026 |
| ONU Services Limited | Orpington | 2018 |
| Onubafruit UK Limited | Faversham | 2019 |
| Onward Fishing Company Limited | Ferriby Road Hessle | 1901 |
| Onward Investment Limited | Hessle | 2010 |
| Onward Marine Ltd | Peterhead | 2014 |
| Onward Star Limited | Bridlington | 2010 |
| Onward Trading Ltd. | Fraserburgh | 2002 |
| Onwards Shellfish Limited | Donaghadee | 2024 |
| Onyx (Eco) Ltd | Derby | 2026 |
| Onyx Offshore Limited | Banff | 2019 |
| Oopsy Daisy Day Care Ltd | London | 2010 |
| OOR Lures Limited | Glasgow | 2010 |
| Opal Oak Ltd | Marylebone | 2024 |
| Open Air Dairy Limited | Dorchester | 2017 |
| Open Journey Limited | Bolton | 2018 |
| Open Spaces Landscape and Arboricultural Consultants Limited | Chelmsford | 2008 |
| Openfield Agriculture Limited | Grantham | 1998 |
| Openfield Grain Limited | Grantham | 1994 |
| Openfield Marketing Limited | Grantham | 2002 |
| Operation Cairngorms CIC | Aboyne | 2021 |
| Operation Granite Stone Limited | Scarborough | 2013 |
| Oppenheim Thoroughbred Consulting Ltd | Aberfeldy | 2003 |
| Opperman Plants Limited | Leicester | 1990 |
| Oppo's Working Care Farm Limited | Reading | 2018 |
| Opticell Limited | Saffron Walden | 2025 |
| Optifarm Ltd | Great Chesterford | 2018 |
| Optigro Ltd | Shifnal | 2018 |
| Optimal Natural Agricultural Productivity and Utilisation Ltd | London | 2025 |
| Optimum Lighthouse Ltd | Raunds | 2019 |
| Optimum Tree Services Ltd | Brentwood | 2019 |
| Optimus Stables Ltd | Arundel | 2023 |
| Optivax Ltd | Muir Of Ord | 2025 |
| Optoswim Technologies Ltd | Glasgow | 2007 |
| Opulence Thoroughbreds Breeding Limited | Newmarket | 2022 |
| Opus Agencies Limited | Leamington Spa | 1993 |
| Oran Boyd Ltd | Isle Of Barra | 2023 |