| Norton Sixteen Limited | Wholesale & retail | 2024 |
| Notcoi Business Solutions Limited | Hospitality | 2006 |
| Notre Dame Investments Ltd | Real estate | 2018 |
| Novastron Advisory Limited | Professional & scientific | 2025 |
| NOX Gymnastics Ltd | Arts & recreation | 2019 |
| NS Alpha Company Ltd | Information & communication | 2023 |
| NS Insulation Products Ltd | Construction | 2016 |
| NT Leadership Limited | Real estate | 2024 |
| Nuclear Support Services 2025 Limited | Administrative & support | 2025 |
| Nuclear Support Services Limited | Administrative & support | 2024 |
| NWB Consultancy Limited | Professional & scientific | 2025 |
| O E S Medical Limited | Manufacturing | 1995 |
| O'Dwyer Direct Search Limited | Administrative & support | 2013 |
| O. B. E. Properties Ltd | Real estate | 2009 |
| O.G. Stonemasonry Contractors Limited | Construction | 2003 |
| OA Construction Limited | Construction | 2018 |
| Oak Court 69 Black Bourton Road Carterton Ltd | Households | 2015 |
| Oak Grove Associates Ltd | Professional & scientific | 2013 |
| Oak Island Films Ltd | Information & communication | 2017 |
| Oakarch Limited | Professional & scientific | 1987 |
| Oakfield Place Management Company Limited | Households | 2000 |
| Oakfield Vigilance Ltd | Administrative & support | 2021 |
| Oakford Carpentry Limited | Construction | 2019 |
| Oakki Properties Limited | Real estate | 2023 |
| Oakridge Farms (Dorset) Limited | Agriculture & fishing | 2009 |
| Oakridge Farms (Holdings) Limited | Professional & scientific | 2009 |
| Oaks Joinery Limited | Construction | 2014 |
| OAS Cars Limited | Wholesale & retail | 2018 |
| OB Developments Limited | Manufacturing | 2006 |
| OC I Engco Limited | Financial services | 2024 |
| Ocea UK Limited | Wholesale & retail | 2009 |
| Ocean Decks Ltd | Construction | 2025 |
| Ocean Link Forwarders Limited | Other services | 2018 |
| Ocie Investment Group Limited | Construction | 2018 |
| OCK Street Management Company Limited | Households | 2000 |
| Ocomputers Ltd | Information & communication | 2019 |
| Octa Capital (Churchill House) Ltd | Construction | 2026 |
| Octa Capital (D’Oilly Lodge) Ltd | Construction | 2026 |
| Octa Capital (D’Oilly Mews) Ltd | Construction | 2026 |
| Octa Capital (Langley House) Ltd | Construction | 2026 |
| Octa Capital (Sturt Farmhouse) Ltd | Construction | 2026 |
| Octa Capital (Sturt Lodge) Ltd | Construction | 2026 |
| Octa Capital Ltd | Financial services | 2023 |
| Oculista Ltd | Health & social care | 2024 |
| Ocus Wholesale Ltd | Wholesale & retail | 2022 |
| Odcc Clubhouse Limited | Hospitality | 2019 |
| Oddball Motors Limited | Wholesale & retail | 2012 |
| OFF the Hook (Hangborough) Limited | Hospitality | 2024 |
| Ofip Limited | Professional & scientific | 2020 |
| Ofui Limited | Professional & scientific | 2019 |
| Ogilvie Enterprises Ltd | Transport & logistics | 2024 |
| Ohutu Limited | Information & communication | 2018 |
| Olaray Ltd | Professional & scientific | 2023 |
| Old Abbey House Hotel Limited | Hospitality | 2023 |
| Old Forge Pottery Limited | Arts & recreation | 2024 |
| Old Orchard Court (Witney) Management Company Limited | Households | 2021 |
| Old SOW Productions Limited | Information & communication | 2020 |
| Olek Ciupa Painter Ltd | Administrative & support | 2016 |
| Olidai Consulting Ltd | Professional & scientific | 2024 |
| Oliveri Antiques Limited | Wholesale & retail | 1995 |
| OMC Global Limited | Transport & logistics | 2013 |
| OMG Investments Ltd | Financial services | 2019 |
| OMK Design Consultancy Limited | Professional & scientific | 2006 |
| OMK Property Limited | Financial services | 2023 |
| Omnia Energy Limited | Administrative & support | 2025 |
| Omnia-Enterprises Limited | Wholesale & retail | 2020 |
| Omsuk Ltd | Information & communication | 1996 |
| ON the Fret Ltd | Information & communication | 2024 |
| One Folly Bridge Limited | Real estate | 1997 |
| One HAT Design Limited | Professional & scientific | 1998 |
| One Health Limited | Health & social care | 2016 |
| One Million Worlds Limited | Information & communication | 2018 |
| One More Roof Limited | Financial services | 2014 |
| One Stop Automation Limited | Professional & scientific | 2004 |
| One Witney Limited | Real estate | 2020 |
| One YOU Evolve Ltd | Professional & scientific | 2023 |
| Only Essentials Ltd | Wholesale & retail | 2020 |
| Only Holes Limited | Manufacturing | 2025 |
| Onlyarchive Ltd | Professional & scientific | 2023 |
| Onyx Build Ltd | Construction | 2022 |
| Opalex Limited | Construction | 2004 |
| Open Doors With Brother Andrew | Other services | 2007 |
| Opera Rose Pocket Studio Ltd | Arts & recreation | 2022 |
| Opted Limited | Manufacturing | 2022 |
| Optia Data Ltd | Information & communication | 2024 |
| Optimise Property Ltd | Real estate | 2019 |
| Optimum Health Care Solutions Ltd | Health & social care | 2022 |
| OPUS3 Aromas Limited | Wholesale & retail | 2025 |
| Orange Landscapes Ltd | Administrative & support | 2024 |
| Orcc Trading Ltd | Wholesale & retail | 2010 |
| Orchard Barn SPV Ltd | Real estate | 2025 |
| Orchid House (Stonesfield) Management Company Limited | Households | 2024 |
| ORK Consulting Limited | Professional & scientific | 2019 |
| Orlen Holdings Limited | Professional & scientific | 2025 |
| Ormrod Building Services Limited | Construction | 2017 |
| OSA Systems Ltd | Manufacturing | 2019 |
| Oska Johansson Ltd | Wholesale & retail | 2019 |
| Osney Capital I GP LLP | | 2024 |
| Osney Capital I LP | | 2024 |
| Osney Capital LLP | | 2024 |