| NINETEEN57 Hairdressing Co. Ltd | Other services | 2025 |
| Nitrous Group Limited | Administrative & support | 2018 |
| Njfmj Investments Ltd | Financial services | 2022 |
| NO Ltd | Extraterritorial | 2025 |
| NO.22 Church Street Limited | Households | 1976 |
| Noble Home Solutions Ltd | Construction | 2025 |
| Noel Partnerships Limited | Construction | 1996 |
| Noffle Studios Ltd | Manufacturing | 2021 |
| Noor Khadijah Limited | Health & social care | 2026 |
| Norcross Consulting Ltd | Administrative & support | 2026 |
| Norse Freedom From Pain Clinic Ltd | Health & social care | 2018 |
| Norse Personal Training Ltd | Arts & recreation | 2018 |
| Norse Property Investments Ltd | Real estate | 2021 |
| North Hall Farm Barns Ltd | Households | 2013 |
| North Star Mentoring Limited | Education | 2026 |
| North Street Barbers Ltd | Other services | 2019 |
| North Uttlesford PCN Limited | Health & social care | 2022 |
| Northwood Carter Ltd | Professional & scientific | 2004 |
| Norwood Construction Group Limited | Water & waste | 2021 |
| Nothing Safe Limited | Administrative & support | 2021 |
| Novacom Digital Limited | Extraterritorial | 2010 |
| Novacom Group Limited | Administrative & support | 2010 |
| Novacom Limited | Extraterritorial | 2020 |
| Novacom London Limited | Administrative & support | 2014 |
| NRG Rail Ltd | Construction | 2026 |
| NT Ltd | Extraterritorial | 2009 |
| Nudge Graphics Limited | Professional & scientific | 2014 |
| Nugene Ltd | Manufacturing | 1992 |
| Nugent Asset Management Limited | Real estate | 2013 |
| Numbercloud Limited | Professional & scientific | 2013 |
| NUT Hills Ltd | Manufacturing | 2021 |
| Nutrio Limited | Professional & scientific | 2012 |
| NW6 Self Storage Ltd | Real estate | 2022 |
| Nwdwm Ltd | Financial services | 2026 |
| NZ Investment Ltd | Real estate | 2020 |
| O'Keefe Construction & Development Ltd | Construction | 2025 |
| Oakbridge International Limited | Transport & logistics | 1995 |
| Oakhurst Trading Limited | Financial services | 2026 |
| Oasis Engineering Services Limited | Energy | 1997 |
| Obsidian Eyewear Ltd | Wholesale & retail | 2024 |
| Occam Trading & Technology Consulting Limited | Professional & scientific | 2011 |
| Oculus Knowledge Consulting Ltd | Professional & scientific | 2023 |
| Odysseos Limited | Hospitality | 2020 |
| OE Ltd | Extraterritorial | 2008 |
| OFF Market Consultancy Solutions Limited | Other services | 2016 |
| OG Ltd | Extraterritorial | 2008 |
| OIL Trading Solutions Limited | Professional & scientific | 2026 |
| OJW Enterprises Limited | Wholesale & retail | 2016 |
| Old Forge Property Management Ltd | Real estate | 2021 |
| Old Rowley Limited | Real estate | 2026 |
| Olitool Ltd | Information & communication | 2019 |
| Olive & Olive Limited | Wholesale & retail | 2021 |
| Olive Branch Consulting Ltd | Professional & scientific | 2024 |
| Olive Tree 424 Ltd | Real estate | 2011 |
| Olivers Lodge Limited | Administrative & support | 2002 |
| OM Ceylon Limited | Administrative & support | 2023 |
| Onaria Technologies Ltd | Professional & scientific | 2015 |
| Oncore Talent Limited | Arts & recreation | 2020 |
| One Day Like This Ltd | Professional & scientific | 2024 |
| ONEHUNDRED&5 Limited | Health & social care | 2026 |
| Onion Smash Burger Ltd | Hospitality | 2024 |
| ONL Audio Ltd | Wholesale & retail | 2018 |
| Only Trans Tech Limited | Wholesale & retail | 2025 |
| ONN Studio Limited | Professional & scientific | 2014 |
| OP Ltd | Extraterritorial | 2009 |
| OP6 Innovation Limited | Professional & scientific | 2016 |
| Operaglass Works Ltd | Arts & recreation | 2016 |
| OPS AI Limited | Administrative & support | 2025 |
| Opsys Solutions Ltd | Information & communication | 2025 |
| Opticell Limited | Agriculture & fishing | 2025 |
| Opticspr Ltd | Information & communication | 2020 |
| Optimization Services Limited | Professional & scientific | 2019 |
| Optio Pizza Ltd | Hospitality | 2023 |
| Option Solutions LLP | | 2013 |
| Opulence Distribution Limited | Wholesale & retail | 2022 |
| Opulent Consultancy Group Ltd | Professional & scientific | 2025 |
| OQK Associates Limited | Administrative & support | 2016 |
| Orange Direct Ltd | Professional & scientific | 2018 |
| Orbus Consulting Services Limited | Other services | 2008 |
| Orchard Properties (Maidenhead) Ltd | Real estate | 2019 |
| Orditum Ltd | Professional & scientific | 2025 |
| Organic Countryside Community Interest Company | Agriculture & fishing | 2009 |
| Organic Orchards Limited | Agriculture & fishing | 2014 |
| Organic Photovoltaics Ltd | Professional & scientific | 2011 |
| Originex Limited | Professional & scientific | 2010 |
| Origo Management Limited | Professional & scientific | 2010 |
| Orthotechs Limited | Health & social care | 2023 |
| Osborn Interiors Ltd. | Professional & scientific | 2012 |
| Osborne Housing Services B Limited | Real estate | 2021 |
| Osborne Housing Services Limited | Professional & scientific | 2020 |
| Oshar Limited | Real estate | 2025 |
| OT Chilli Ltd | Wholesale & retail | 2021 |
| Ottery Four Limited | Real estate | 2019 |
| OUR Culture MAG Limited | Information & communication | 2017 |
| Ourpinboard Limited | Information & communication | 2014 |
| Outside-In Solutions (Design & Build) Limited | Construction | 2012 |
| Overdog Ltd | Professional & scientific | 2018 |
| Oxford Sinfonia Ltd | Arts & recreation | 2017 |
| Oyster Nutrition Ltd | Health & social care | 2026 |
| P D E Engineering Limited | Construction | 2012 |