| Old Market Barbers HFD Ltd | Other services | 2025 |
| Old School Investments Ltd | Financial services | 2025 |
| Old Star & Garter Limited | Households | 1982 |
| Oldaker & Sons Ltd | Transport & logistics | 2019 |
| Oliver's Mobile Dog Gym Ltd | Other services | 2025 |
| Ollie Anderson Car Sales Ltd | Wholesale & retail | 2022 |
| Ollie Anderson Holdings Ltd | Financial services | 2022 |
| Olpass Limited | Professional & scientific | 2008 |
| Olusap Ltd | Real estate | 2025 |
| Olympus Electrical Solutions Ltd | Construction | 2022 |
| Olympus Energy Independence Ltd | Construction | 2025 |
| Omag Ltd | Wholesale & retail | 2008 |
| Omega K9 Ltd | Other services | 2015 |
| Omega Solutions Property Limited | Real estate | 2019 |
| Omega Web Design Limited | Information & communication | 2021 |
| Omni Power Engineering Limited | Construction | 2025 |
| Omnianda Limited | Professional & scientific | 2023 |
| Omnitec Consulting Ltd | Information & communication | 2015 |
| ON Point Cider Company Ltd | Manufacturing | 2023 |
| ON Site Welding Solutions Ltd | Manufacturing | 2004 |
| Onagen Limited | Professional & scientific | 2025 |
| One 2 One VA Limited | Public administration | 2024 |
| One Garratt Lane Ltd | Real estate | 2025 |
| One Life Medical Limited | Health & social care | 2023 |
| One Tandem Limited | Information & communication | 2026 |
| One-Eyed Dog Limited | Arts & recreation | 1990 |
| Onemd Consulting Limited | Health & social care | 2025 |
| Onemd Ltd | Wholesale & retail | 2024 |
| Oneself Psychology Ltd | Health & social care | 2026 |
| OP Software Limited | Information & communication | 2010 |
| OPA Investments Ltd | Real estate | 2017 |
| Opal Lake Ltd | Manufacturing | 2023 |
| Open Sky Productions Ltd | Information & communication | 2012 |
| Open Sky Theatre | Arts & recreation | 2018 |
| Open UP (UK) Limited | Information & communication | 2013 |
| Openhighstreet Limited | Wholesale & retail | 2010 |
| Opsec Management Ltd | Professional & scientific | 2023 |
| Opspilot Ltd | Information & communication | 2026 |
| Optimal Oak Ltd | Construction | 2026 |
| Optimal Performance 24 Ltd | Arts & recreation | 2024 |
| Opulent Construction Ltd | Construction | 2014 |
| Oranda Solutions Limited | Construction | 2004 |
| Orbit Hoist Design Ltd | Professional & scientific | 2019 |
| Orbiting Books Ltd | Wholesale & retail | 2005 |
| Orchard Close Glewstone Management Company Limited | Households | 2021 |
| Orchard Fundraising Limited | Administrative & support | 2016 |
| Orchard Hope Ltd | Professional & scientific | 2026 |
| Orchard Multi-Academy Trust | Education | 2011 |
| Orchard Sage Agricultural Ltd | Agriculture & fishing | 2025 |
| Orchard Sage Property Ltd | Real estate | 2024 |
| Orchards & Mill Ltd | Real estate | 2024 |
| Orcop Osteopathic Practice Limited | Health & social care | 2007 |
| Orcop Treetops Limited | Agriculture & fishing | 2021 |
| Ordinges Place Management Limited | Households | 2024 |
| ORG Investments 1 Limited | Real estate | 2017 |
| Organic Properties Limited | Real estate | 2013 |
| Oriental Planet Limited | Hospitality | 2025 |
| Origin Pizza Ltd | Hospitality | 2019 |
| Originpin Ltd | Information & communication | 2026 |
| Orphans House 22 Limited | Real estate | 2022 |
| Oryx Consulting Limited | Professional & scientific | 2026 |
| Oryx Property Investments Limited | Real estate | 2022 |
| Oryx Property Management Limited | Real estate | 2023 |
| Oscar Leadon Limited | Real estate | 2016 |
| Osprey Syndicate Ltd | Construction | 2018 |
| Oswald RD Management Company Limited | Households | 2018 |
| OT Business Consultancy Limited | Professional & scientific | 2025 |
| Ot&B Ltd | Real estate | 2021 |
| OTM Biochar Ltd | Extraterritorial | 2024 |
| OTM Groundscare Ltd | Administrative & support | 2016 |
| OTS Limited | Wholesale & retail | 1996 |
| Otstores Limited | Extraterritorial | 1996 |
| Otter-Barry Books Limited | Information & communication | 2015 |
| Ottoman & Oak Ltd | Wholesale & retail | 2025 |
| OUR Curiosity Corner Ltd | Health & social care | 2025 |
| OUR Therapy and Mediation Ltd | Health & social care | 2025 |
| Ourpulse Limited | Other services | 2023 |
| OUT There Consulting & Management Ltd | Professional & scientific | 2025 |
| Outback Air Ltd | Arts & recreation | 2016 |
| Outback International (UK) Limited | Wholesale & retail | 2013 |
| Outdoor Ways CIC | Other services | 2026 |
| Outlaws Coffee Ltd | Professional & scientific | 2019 |
| Outrospective Design Studio Ltd | Professional & scientific | 2023 |
| Outstanding Education Solutions Ltd | Administrative & support | 2013 |
| Over a Cuppa Ltd | Real estate | 2021 |
| Owen Cattle Co. Limited | Agriculture & fishing | 2012 |
| Owen Hicks Architecture Ltd | Professional & scientific | 2013 |
| Owen Pell Limited | Construction | 1976 |
| Owens and Box Ltd | Wholesale & retail | 2017 |
| Owens Cross Developments Limited | Construction | 2024 |
| Oxfork Limited | Hospitality | 2025 |
| OZF Consulting Limited | Other services | 2014 |
| Ozone Global Security Ltd | Administrative & support | 2013 |
| P & a Lloyd Consulting Limited | Professional & scientific | 2005 |
| P & D Green Waste Limited | Water & waste | 2014 |
| P & F Eggs Limited | Agriculture & fishing | 2004 |
| P a Investments Limited | Real estate | 2017 |
| P and R Design Limited | Professional & scientific | 1997 |
| P B Yeomans Agricultural Contractor Limited | Construction | 2007 |
| P H Financial Management Limited | Professional & scientific | 2001 |