| Nude Theory Ltd | Arts & recreation | 2025 |
| Nuffield Land Limited | Real estate | 2011 |
| Nugen Construction Ltd | Construction | 2024 |
| Nugen Consultants Ltd | Professional & scientific | 2020 |
| Nugen Services Ltd | Real estate | 2018 |
| Number One (GB) Limited | Other services | 2009 |
| Numberfit Limited | Education | 2012 |
| Nuprod Limited | Wholesale & retail | 1995 |
| Nurbhai Enterprises Holdings Limited | Financial services | 2025 |
| Nurscare Health Limited | Administrative & support | 2017 |
| Nurse at Home Ltd | Administrative & support | 2020 |
| Nursery Court Residents Association Limited | Real estate | 1988 |
| Nutricloset Ltd | Information & communication | 2024 |
| Nutrition by Beenish Ltd | Manufacturing | 2025 |
| Nutrizest Ltd | Health & social care | 2020 |
| Nuttall Brothers Limited | Real estate | 2024 |
| Nuvant IT Limited | Information & communication | 2026 |
| NVQ Construction Skills Limited | Education | 2021 |
| NW Driving Academy Ltd | Education | 2026 |
| NXF Ltd | Administrative & support | 2024 |
| NXR Group Ltd | Wholesale & retail | 2015 |
| Nxtalps Ltd | Information & communication | 2022 |
| Nyco Limited | Information & communication | 1988 |
| Nyco Solutions Limited | Information & communication | 2016 |
| NYN Optical Ltd | Health & social care | 2019 |
| Nyra Homes Limited | Real estate | 2023 |
| Nyra Imports Ltd | Wholesale & retail | 2015 |
| O Hara Agricultural-Workshop Maintenance Ltd | Manufacturing | 2023 |
| O Sabor DO Brasil Ltd | Hospitality | 2026 |
| O&P Glass Ltd | Construction | 2005 |
| O'Brien Construction Services Limited | Professional & scientific | 2015 |
| O'Sullivan Civil & Construction Limited | Construction | 2025 |
| O.K.& J (London) Ltd | Construction | 2020 |
| O.S. Enterprises Limited | Wholesale & retail | 2009 |
| Oak and Moss Ltd | Information & communication | 2010 |
| Oak Haven Support Services Ltd | Health & social care | 2025 |
| Oak Specialist Services Limited | Construction | 2005 |
| Oakford Digital Limited | Information & communication | 2025 |
| Oakhouse Foods Limited | Wholesale & retail | 2001 |
| Oakleaf Care (Hartwell) Limited | Health & social care | 2004 |
| Oakleigh (UK) Limited | Wholesale & retail | 2007 |
| Oaklite Ltd | Real estate | 2020 |
| Oakman Architectural Consultants Ltd | Professional & scientific | 2022 |
| Oakman Lord Ltd | Professional & scientific | 2023 |
| Oakridge Court Residents (High Wycombe) Limited | Real estate | 1992 |
| Oakridge Prestige Limited | Wholesale & retail | 2012 |
| Oakshott Solutions Limited | Wholesale & retail | 2017 |
| Oakside Travel Limited | Wholesale & retail | 2024 |
| Oakstone Real Estate Ltd | Real estate | 2023 |
| Oaktree Property Company Limited | Financial services | 2009 |
| Oakview House Management Co Ltd | Extraterritorial | 2014 |
| Oakwood Housing Limited | Hospitality | 2003 |
| Oasis Play Ltd | Arts & recreation | 2022 |
| Oasis Play Nurseries Ltd | Health & social care | 2025 |
| OBC Commercial Limited | Real estate | 2023 |
| Oberlin Healthcare Private Limited | Extraterritorial | 2016 |
| Obsidianpriest Ltd | Wholesale & retail | 2026 |
| Ocean Baby Ltd | Wholesale & retail | 2020 |
| Oceanic Environmental Cables Group Limited | Extraterritorial | 2020 |
| Oceanteam Logistics International Limited | Transport & logistics | 2016 |
| OCG Lighting Technology Limited | Manufacturing | 2016 |
| Octalink UK Ltd | Hospitality | 2025 |
| Octillion Academy Ltd | Education | 2024 |
| Octillion Training Academy Limited | Education | 2017 |
| Octisage Limited | Transport & logistics | 2003 |
| Octo Property Management Limited | Real estate | 2026 |
| Octopi Gold Limited | Wholesale & retail | 2018 |
| Octopus Homes Limited | Real estate | 2021 |
| OD Desserts Ltd | Hospitality | 2023 |
| Odditea Ltd | Wholesale & retail | 2025 |
| OE Electrical Services Ltd | Construction | 2024 |
| Ofeh Technologies Limited | Professional & scientific | 2022 |
| Office & General Group Limited | Professional & scientific | 2006 |
| Office Furniture Services Ltd | Construction | 2017 |
| Office Homes Limited | Hospitality | 2019 |
| Office Safe International Limited | Professional & scientific | 2025 |
| Ogami Solanki Ltd | Professional & scientific | 2020 |
| Ohaana Limited | Hospitality | 2021 |
| OI Group Limited | Information & communication | 2026 |
| OIC Sales Ltd | Manufacturing | 2024 |
| Ojais Wellness Ltd | Other services | 2024 |
| OJM Consulting Ltd | Information & communication | 2023 |
| OK Hanna Ltd | Health & social care | 2020 |
| OK Proca Ltd | Information & communication | 2024 |
| OKV PVT Limited | Education | 2013 |
| Okwild Ltd | Construction | 2018 |
| Okyes Ltd | Real estate | 2024 |
| Old Chase Farm Ltd | Real estate | 2018 |
| Old Oak Engineering Ltd | Professional & scientific | 2022 |
| Old Park Development Ltd | Extraterritorial | 2022 |
| Old School Caretaker Limited | Administrative & support | 2023 |
| Oleander House Development Limited | Households | 2023 |
| Oleksiyenko Ltd | Wholesale & retail | 2023 |
| Olerio Sports Management Ltd | Arts & recreation | 2025 |
| Oli's Cleaning Limited | Administrative & support | 2022 |
| Olib Ltd | Wholesale & retail | 2025 |
| Olive Partners Limited | Real estate | 2020 |
| Olive Road (Ealing) Management Company Limited | Households | 1997 |
| Oliver D Winter Ltd | Administrative & support | 2015 |
| Oliver Prestige Motors Limited | Wholesale & retail | 2023 |