| NL Care Gloucester Limited | Construction | 2019 |
| NL Care Maidstone Limited | Construction | 2026 |
| NL Care Properties Ltd | Construction | 2021 |
| NL Care Property Limited | Extraterritorial | 2018 |
| NLC Buildings Limited | Construction | 2021 |
| NLP Rickmansworth Limited | Construction | 2020 |
| NM Speech Therapy Ltd | Health & social care | 2025 |
| Nmcz Property Limited | Real estate | 2022 |
| Nobl Q UK Ltd | Information & communication | 2019 |
| Nolan Care Consultancy Ltd | Health & social care | 2019 |
| Norman a. Peroni (Holdings) Limited | Professional & scientific | 2004 |
| Norman a. Peroni Limited | Administrative & support | 1989 |
| North Harrow Waltonians Angling Society | Agriculture & fishing | 2007 |
| North of Paris Limited | Manufacturing | 2018 |
| North Western Avenue Watford Limited | Construction | 2014 |
| Northbridge Transformation Ltd | Professional & scientific | 2026 |
| Nouvelle Health Limited | Administrative & support | 2024 |
| Nova Seza Limited | Manufacturing | 2026 |
| Novara Consultancy Ltd | Education | 2019 |
| Novetal Ltd | Information & communication | 2008 |
| Nowca Limited | Arts & recreation | 2011 |
| Nowus Healthcare Ltd | Wholesale & retail | 2022 |
| NP Legal Limited | Professional & scientific | 2024 |
| NR Flowers UK Ltd | Extraterritorial | 2022 |
| NRI Services UK Ltd | Administrative & support | 2023 |
| NSY Techs and Estates Limited | Information & communication | 2014 |
| Nurton & Thompson Limited | Real estate | 2003 |
| Nutrition for Energy Ltd | Education | 2021 |
| Nwspk Ltd | Information & communication | 2020 |
| O&M Studio Ltd | Professional & scientific | 2025 |
| O.S Renewable Services Ltd | Manufacturing | 2022 |
| Oakgrove Properties Ltd | Real estate | 2025 |
| Oakridge Developments (JN) Ltd | Construction | 2025 |
| Oakstone Consultancy Ltd | Administrative & support | 2025 |
| Oakstone Investment Group Ltd | Real estate | 2025 |
| Oasis Inclusion Academy Limited | Education | 2010 |
| Object Professionals Limited | Information & communication | 2001 |
| Obsequella Ltd | Professional & scientific | 2021 |
| Obsidian Artworks Limited | Professional & scientific | 2013 |
| Octavius GB Limited | Financial services | 2013 |
| Odle Enterprises Ltd | Health & social care | 2007 |
| OE Auto Solutions Ltd | Wholesale & retail | 2023 |
| Offshoot Arts Ltd | Wholesale & retail | 2022 |
| Offshore Rope Services Limited | Construction | 2011 |
| Offspring Design Ltd | Wholesale & retail | 2010 |
| Olive Print and Graphics Ltd | Manufacturing | 2015 |
| Oliver Hive Ltd | Administrative & support | 2018 |
| Oliveros Events Ltd | Hospitality | 2023 |
| Oliveros Ltd | Hospitality | 2022 |
| Ollie Roberts Limited | Hospitality | 2024 |
| OM Metropolitan Total Security Limited | Administrative & support | 2004 |
| OM Security Agency Limited | Administrative & support | 2008 |
| OM Tech ST Ltd | Wholesale & retail | 2019 |
| Omaq Display Delivery Ltd | Manufacturing | 2023 |
| Omega Beauty and Hair Salon Ltd | Other services | 2019 |
| Omega Integrated Care Limited | Health & social care | 2018 |
| Omega Properties and Investments Ltd | Financial services | 2019 |
| Ommani Ltd | Wholesale & retail | 2025 |
| Omnia Personal Assistance Ltd | Other services | 2019 |
| ON Trend Products Ltd | Wholesale & retail | 2026 |
| On-Court Ltd | Wholesale & retail | 2023 |
| Oncovery Ltd | Professional & scientific | 2018 |
| One Design Studio Limited | Professional & scientific | 2000 |
| One Realty Ltd | Real estate | 2021 |
| One Stop Consultants Ltd | Information & communication | 2006 |
| One Stop Motor Services Ltd | Wholesale & retail | 2011 |
| Ontologika Ltd | Professional & scientific | 2021 |
| Ooss (UK) Limited | Extraterritorial | 2016 |
| Optimal Consulting Ltd | Information & communication | 2001 |
| Orca Design Limited | Administrative & support | 2003 |
| Orelis London Ltd | Wholesale & retail | 2025 |
| Organisation Performance Limited | Information & communication | 2003 |
| Oriental Health & Care Limited | Health & social care | 2004 |
| Origin Evidence Limited | Professional & scientific | 2023 |
| Origin Strategic Communications Limited | Professional & scientific | 2024 |
| Original Clarity Ltd | Professional & scientific | 2025 |
| Original RM Group Limited | Financial services | 2022 |
| Original RM Limited | Professional & scientific | 2017 |
| Orion Recruitment Limited | Administrative & support | 2010 |
| Orion Trailers Ltd | Wholesale & retail | 2024 |
| Orla Consulting Limited | Administrative & support | 2014 |
| Ormer Consulting Ltd | Information & communication | 2019 |
| OS Refinish Ltd. | Manufacturing | 2016 |
| Oskoreia Digital Systems and Media Ltd | Information & communication | 2024 |
| Oslar Design Limited | Information & communication | 2014 |
| Osteobiz Limited | Administrative & support | 2019 |
| Otam Limited | Health & social care | 2023 |
| OUR Cognitive Solutions Ltd | Wholesale & retail | 2019 |
| Outside Broadcast's Services Ltd | Information & communication | 2013 |
| Over & Above Construction Ltd | Construction | 2021 |
| OWL Real Property Limited | Real estate | 2025 |
| Ownistry Ltd | Information & communication | 2025 |
| Oxfordshire Narrowboats Limited | Hospitality | 1987 |
| P & H Ventures Limited | Education | 2010 |
| P & P Cregan Limited | Transport & logistics | 2005 |
| P and M Services Amersham Limited | Extraterritorial | 2010 |
| P E Norman Limited | Construction | 2014 |
| P J Green Holdings Limited | Construction | 2023 |
| P J Hodges Ltd | Manufacturing | 2014 |
| P Jarrott Decorating Ltd | Construction | 2016 |