| NV Infotek Limited | Information & communication | 2025 |
| NVA Property Limited | Real estate | 2021 |
| NVP Healthcare Consulting Limited | Professional & scientific | 2012 |
| NVP Properties Ltd | Real estate | 2011 |
| NW Property Management Limited | Administrative & support | 2003 |
| NWL PUB Co Ltd | Hospitality | 2023 |
| Nyla TP Limited | Real estate | 2021 |
| Nzuri Limited | Wholesale & retail | 2016 |
| O Brien Scenery Ltd | Construction | 2025 |
| O'Donnell Motors Ltd | Wholesale & retail | 2025 |
| O'Hara Bros. Services Limited | Professional & scientific | 2016 |
| O'Hara Bros. Surfacing Holdings Limited | Financial services | 2018 |
| O'Hara Transport Limited | Real estate | 1997 |
| O'Kane Building Limited | Construction | 2004 |
| O'Shea Contractors Ltd | Construction | 2025 |
| O.S Barnes Limited | Construction | 2016 |
| O.T.A. Investments Limited | Real estate | 2003 |
| Oak and Smoke Ltd | Manufacturing | 2014 |
| Oak Lodge (Hatch END) Limited | Households | 1997 |
| Oakdale Financial Services Ltd | Financial services | 2011 |
| Oakdale Investment Services Ltd | Real estate | 2026 |
| Oakdale Property Services Ltd | Real estate | 2019 |
| Oakeve Landscaping Limited | Administrative & support | 2008 |
| Oakfield Care Home Ltd | Professional & scientific | 2014 |
| Oakhill (Southern) Limited | Administrative & support | 2002 |
| Oakleigh Group of Pharmacies Limited | Wholesale & retail | 1992 |
| Oakleigh N20 Limited | Real estate | 2018 |
| Oakroots Cleaning Clearance Ltd | Administrative & support | 2025 |
| Obellon Consulting Ltd | Professional & scientific | 2025 |
| OBI Consultancy Limited | Transport & logistics | 2020 |
| Ockley Investment Limited | Professional & scientific | 2026 |
| Octagon Travel Ltd | Administrative & support | 2025 |
| ODO Services Ltd | Construction | 2015 |
| Ogbegems Ltd | Wholesale & retail | 2024 |
| OIL & GAS Management Limited | Professional & scientific | 1995 |
| OJL (Retail and Media) Ltd | Wholesale & retail | 2011 |
| Okan Limited | Construction | 2007 |
| Okhai & Co Ltd | Professional & scientific | 2021 |
| Ol&P Limited | Information & communication | 2021 |
| Old Barn Consulting Limited | Professional & scientific | 2018 |
| Old Hall Ventures Limited | Financial services | 2025 |
| Old Lyonian Athletic Club Limited(The) | Professional & scientific | 1921 |
| Old Rectory Care Ltd | Health & social care | 2018 |
| Oldknow Properties Ltd | Real estate | 2018 |
| Oleander Enterprises Limited | Professional & scientific | 2025 |
| Oliver Dale Property Services Limited | Real estate | 2023 |
| Oliver Medcare Ltd | Health & social care | 2019 |
| OM Hanumate Limited | Real estate | 1995 |
| OM Medical Limited | Wholesale & retail | 2010 |
| Omarr Ltd | Wholesale & retail | 2019 |
| OMB Construction Ltd | Construction | 2019 |
| OMB Event Organisers Ltd | Other services | 2024 |
| OMB Eximp Ltd | Wholesale & retail | 2026 |
| Omkar Religious and Astrology Services Limited | Other services | 2024 |
| Omnific Ltd. | Real estate | 2007 |
| Omnivista Glass Ltd | Construction | 2019 |
| Omniya Reliance Capital Limited | Financial services | 2022 |
| Omniya Reliance Limited | Financial services | 2019 |
| Omsai Atelier Ltd | Wholesale & retail | 2026 |
| ON a Stick Limited | Hospitality | 2024 |
| ON Call Accounting Ltd | Information & communication | 2009 |
| On-Site Assessment Limited | Professional & scientific | 2018 |
| Onco Group Limited | Wholesale & retail | 2016 |
| Oncomedica Ltd | Health & social care | 2026 |
| One Carpenter Limited | Manufacturing | 2024 |
| One DOT World Ltd | Wholesale & retail | 2017 |
| One O Eight Branding Ltd | Wholesale & retail | 2025 |
| One Prestige Limited | Wholesale & retail | 2022 |
| One Search Banter Limited | Administrative & support | 2007 |
| One the White House Properties Limited | Real estate | 2025 |
| One Thousand Eyes Ltd | Information & communication | 2019 |
| One World Research Services Limited | Professional & scientific | 2009 |
| One-Nil Advisory Limited | Professional & scientific | 2010 |
| Onikolobo Holdings Ltd | Real estate | 2021 |
| Online Expert Ease Limited | Professional & scientific | 2021 |
| Online Options Limited | Wholesale & retail | 2014 |
| Online Prescriptions Limited | Professional & scientific | 2013 |
| Only Dead Fish Ltd | Professional & scientific | 2010 |
| Only END of Tenancy Cleaning Limited | Administrative & support | 2025 |
| Only Freeholds Limited | Real estate | 2017 |
| ONS Property Services Ltd | Real estate | 2014 |
| Onvi Ltd | Real estate | 2020 |
| OOB Consultancy Limited | Wholesale & retail | 2011 |
| Opal Circle Ltd | Administrative & support | 2026 |
| Openchime Property Management Limited | Households | 1990 |
| Operation Raleigh Promotions Limited | Information & communication | 1983 |
| Optiflow Dynamics Limited | Professional & scientific | 2025 |
| Optimal PPC Ltd | Information & communication | 2016 |
| Opulent Rooms Limited | Real estate | 2024 |
| Opulenze Capital Limited | Financial services | 2020 |
| Opus Brickwork Ltd | Construction | 2010 |
| Opus Fine Arts Limited | Manufacturing | 2025 |
| Orange Dog Productions Limited | Administrative & support | 2006 |
| Orange Spaces Ltd | Administrative & support | 2024 |
| ORB Industries Limited | Professional & scientific | 2024 |
| Orbin Kitchen Ltd | Hospitality | 2025 |
| Orbique Limited | Professional & scientific | 2019 |
| Orchard Care (Westbourne) Limited | Health & social care | 2021 |
| Orchard Court (Edgware) Freehold Company Limited | Households | 2021 |
| Orchard Court (Edgware) RTM Company Limited | Real estate | 2005 |