| Northgate Property Partners Ltd | Real estate | 2025 |
| Northstar Ironing Services Ltd | Other services | 2025 |
| Northwest Heating Ltd | Construction | 2024 |
| Northwest London Schools of Taekwondo Ltd | Arts & recreation | 2014 |
| Northwood Hills Properties Limited | Real estate | 2010 |
| Northwood Hills Service Station Limited | Wholesale & retail | 2002 |
| Northwood Hills Tyre & Battery Co. Ltd. | Wholesale & retail | 2008 |
| Nova Software Solutions Ltd | Information & communication | 2024 |
| Novano LUX Drive Ltd | Transport & logistics | 2022 |
| Novarahr Ltd | Information & communication | 2026 |
| Novaticlab UK Ltd | Health & social care | 2021 |
| NPW Consulting Limited | Information & communication | 2011 |
| NQS Valve Seal Co., Ltd | Administrative & support | 2021 |
| NRA 37 Ltd | Transport & logistics | 2025 |
| NRA Educations Ltd | Education | 2021 |
| NRG GAS Solutions Ltd | Construction | 2019 |
| Nrmd Limited | Health & social care | 2013 |
| Nroll Services Limited | Professional & scientific | 2022 |
| NRS Jewellers Limited | Wholesale & retail | 2015 |
| NSK Properties (London) Ltd | Real estate | 2019 |
| Nsrp Care Ltd | Health & social care | 2025 |
| NTH Degree Stunts Limited | Information & communication | 2024 |
| NU Life (UK) Ltd | Extraterritorial | 2013 |
| Nubrake Services Ltd | Wholesale & retail | 2026 |
| Nugget Capital Limited | Other services | 2011 |
| Nursify Training Ltd | Education | 2025 |
| Nuvay Private Ltd | Information & communication | 2021 |
| Nuwave-Media Limited | Wholesale & retail | 2014 |
| NW Civils Limited | Construction | 2021 |
| NW London Investments Ltd | Real estate | 2020 |
| NWL Primary Care Provider Collaborative CIC | Health & social care | 2026 |
| NYA Properties Limited | Real estate | 2018 |
| Nyasaa UK Ltd | Professional & scientific | 2009 |
| NYN Consultants Limited | Information & communication | 2019 |
| O'Driscoll & Sons Limited | Construction | 2010 |
| O'Kelly Limited | Construction | 1995 |
| Oak & Willow Aurum Holdings Limited | Financial services | 2025 |
| Oak and Willow Aurum Ltd | Manufacturing | 2025 |
| Oak Construction and Maintenance Limited | Construction | 2018 |
| Oak Developments & Build Ltd | Construction | 2023 |
| Oak N Brick Property Ltd | Real estate | 2026 |
| Oak Stratton Limited | Other services | 2019 |
| Oakfarm Holdings Limited | Real estate | 2022 |
| Oakland Roofing & Paving Ltd | Construction | 2015 |
| Oaklands Landscapes Limited | Administrative & support | 2016 |
| Oaklands of Rickmansworth (Herts) Limited | Construction | 2022 |
| Oaklands of Rickmansworth Ltd | Administrative & support | 2016 |
| Oaklands Tree Specialists Limited | Agriculture & fishing | 2008 |
| Oakley Consulting Services Ltd | Professional & scientific | 2023 |
| Oakray Contractors Limited | Construction | 2019 |
| Oakwood Haulage & Plant Hire Limited | Administrative & support | 2021 |
| Oakwood Inventories London Ltd | Administrative & support | 2012 |
| Oakwood Tree Surgery Ltd | Construction | 2022 |
| Oasis Enterprises UK Ltd | Real estate | 2009 |
| Obicare Group Ltd | Health & social care | 2022 |
| OBT Properties London Ltd | Real estate | 2025 |
| Ocashproductions Ltd | Information & communication | 2025 |
| Occupy Mars Ltd | Information & communication | 2024 |
| OCR Group Ltd | Transport & logistics | 2021 |
| Octopus Productions Holdings Limited | | 1976 |
| Ocurae Ltd | Professional & scientific | 2015 |
| Oddly Global Ltd | Information & communication | 2021 |
| OFF Peak Break Ltd | Other services | 2022 |
| Officina Macchina Ltd | Wholesale & retail | 2025 |
| Offshore Advertising and Marketing Services Ltd | Wholesale & retail | 2019 |
| OH My CUT Limited | Other services | 2023 |
| OHP Property Ltd | Real estate | 2020 |
| Oiko Revamp Ltd | Construction | 2025 |
| Okra Quality Inspection Ltd | Administrative & support | 2024 |
| Oldbasing Ltd | Wholesale & retail | 2018 |
| Olde English Roofers & General Builders Limited | Construction | 2024 |
| Oldfield Construction (London) Limited | Construction | 2017 |
| Olgopaex Academy Ltd | Information & communication | 2026 |
| OLI Group Ltd | Wholesale & retail | 2021 |
| OLI Richards TV Lighting Design Ltd | Information & communication | 2008 |
| Olienhout Ltd | Wholesale & retail | 2020 |
| Olive Tree Finance Limited | Real estate | 2021 |
| Oliver David Services Ltd | Construction | 2020 |
| Olmek Limited | Construction | 2015 |
| OLY Tech & Industrial Co., Limited | Administrative & support | 2020 |
| OM Builders Limited | Construction | 2025 |
| OM Cash & Carry Ltd | Wholesale & retail | 2018 |
| OM Namo BV Limited | Wholesale & retail | 2022 |
| Oman Rigging.Construction and Services Ltd | Construction | 2023 |
| Omar Romanini Limited | Wholesale & retail | 2021 |
| Omble Estates Ltd | Real estate | 2025 |
| Omdatta Solutions Limited | Information & communication | 2015 |
| Omega Consultancy Services Ltd | Information & communication | 2023 |
| Omega Holdco Ltd | Financial services | 2026 |
| Omega Projects London Ltd | Construction | 2026 |
| Omega Properties One Limited | Real estate | 2022 |
| Omni Hyper Mega Global Limited | Real estate | 2014 |
| ON - Track Partnership Ltd | Health & social care | 2021 |
| ON Facade Limited | Construction | 2014 |
| On-Line Refrigeration & Air Conditioning Limited | Manufacturing | 1996 |
| One Network Properties Ltd | Construction | 2022 |
| One Stop Concierge Ltd | Wholesale & retail | 2025 |
| One Take Stunts Limited | Information & communication | 2021 |
| One Talent Management Ltd | Professional & scientific | 2024 |
| Oneactive Limited | Wholesale & retail | 2025 |