| Niche PDR Ltd | Administrative & support | 2015 |
| Niche PPS LLP | | 2020 |
| Nick Hinge Associates Limited | Professional & scientific | 2001 |
| Nightingale Asset Management Limited | Real estate | 2000 |
| Nihonbo Limited | Professional & scientific | 2001 |
| Nirrie Limited | Professional & scientific | 2021 |
| Nish Bookkeeping Services Limited | Professional & scientific | 2020 |
| NO Ideal State Ltd | Information & communication | 2026 |
| NO Naked Walls Ltd | Wholesale & retail | 2022 |
| NO Spots Left Ltd | Administrative & support | 2024 |
| NO.37 Mortgage & Protection Advice Ltd | Financial services | 2023 |
| Nonna's Authentic Sicilian Products Ltd | Wholesale & retail | 2023 |
| Nonsuch Films Limited | Information & communication | 2001 |
| Nonsuch Mushrooms Limited | Agriculture & fishing | 2021 |
| Norco Property Ltd | Real estate | 2017 |
| Nordelia Limited | Professional & scientific | 2022 |
| Normanhurst Residents Limited | Households | 2016 |
| Northbridge Holdings Limited | Financial services | 2023 |
| Northgate Property Development Limited | Real estate | 2004 |
| Northridge Advisory Limited | Professional & scientific | 2013 |
| NOT Guilty Media Ltd | Professional & scientific | 2025 |
| Nothingfishy Ltd | Wholesale & retail | 2017 |
| Noushi Sushi Ltd | Hospitality | 2011 |
| Nouvelle Ivoire Ltd | Wholesale & retail | 2023 |
| Novamed (Europe) Limited | Wholesale & retail | 2012 |
| Now-How Limited | Professional & scientific | 2005 |
| Nowfocus Limited | Households | 1999 |
| NS Sands Enterprises Ltd | Professional & scientific | 2023 |
| NTS Trustees Limited | Financial services | 1979 |
| Nubo Drago Foods UK Ltd | Administrative & support | 2018 |
| Nudi London Ltd | Hospitality | 2022 |
| Numerion Software Limited | Information & communication | 2009 |
| Nurturing Nutrition Ltd | Health & social care | 2014 |
| Nuthatch Consulting Limited | Professional & scientific | 2012 |
| Nutra Studio Ltd | Manufacturing | 2025 |
| Nutwood (Residents Management) Limited | Households | 2019 |
| NYE Saunders Limited | Professional & scientific | 2010 |
| O & D Construction Ltd | Construction | 2005 |
| O'Neil Productions Ltd | Arts & recreation | 2011 |
| O.Grass Limited | Construction | 2017 |
| OA Surrey Ltd | Real estate | 2022 |
| Oakhanger Management Company Limited | Other services | 1992 |
| Oakhurst Consultants UK Ltd | Professional & scientific | 2021 |
| Oakhurst Financial Management Limited | Financial services | 2012 |
| Oakwood Niche Limited | Administrative & support | 2011 |
| OB Real Estate Ltd | Households | 2023 |
| Obsidian Impact Investments Limited | Extraterritorial | 2025 |
| Oceanic Software Limited | Information & communication | 2024 |
| Ochre Print Studio Limited | Arts & recreation | 2012 |
| Oculus Partners Ltd | Information & communication | 2023 |
| ODB Contracts Limited | Extraterritorial | 1995 |
| ODB Group Limited | Construction | 1993 |
| ODB Netspace Limited | Extraterritorial | 2003 |
| ODB Workspace Limited | Construction | 2000 |
| ODD 02 Limited | Construction | 2026 |
| ODF Investments Limited | Financial services | 2020 |
| Office Landscapes (Midlands) Limited | Other services | 1974 |
| Og's Burgers Ltd | Hospitality | 2019 |
| Ohm-It Electrical Ltd | Construction | 2020 |
| Ohrh Investment Limited | Construction | 2016 |
| Okees Studio Limited | Wholesale & retail | 2020 |
| Old Chase Consulting Ltd | Professional & scientific | 2021 |
| Olddean Finance Limited | Professional & scientific | 2012 |
| Oliver Bone Graphics Ltd | Professional & scientific | 2026 |
| Olko House Plus Limited | Construction | 2025 |
| Olympia Theatre Limited | Arts & recreation | 2025 |
| One Digital Future Ltd | Information & communication | 2024 |
| One Rehab Ltd | Wholesale & retail | 2010 |
| One Stop Conservation Ltd | Professional & scientific | 2016 |
| One Tech London Ltd | Wholesale & retail | 2021 |
| Onichem Europe Limited | Wholesale & retail | 2010 |
| Opal Neon Limited | Information & communication | 2021 |
| Open Box Ideas Limited | Construction | 2014 |
| Open Vista Investment Management Limited | Professional & scientific | 2016 |
| Opeque Limited | Administrative & support | 2007 |
| Opera Security Services Limited | Administrative & support | 2025 |
| OPO Recycling Ltd | Water & waste | 2026 |
| Opportunity Invest Ltd | Real estate | 2020 |
| Opus Blue Limited | Construction | 2014 |
| Opus Commercial Engineering Services Ltd | Professional & scientific | 2026 |
| Orange Jacket Productions Limited | Administrative & support | 2019 |
| Orchard Health & Medical Ltd | Health & social care | 2025 |
| Orenda Air Services Ltd | Wholesale & retail | 2024 |
| ORG Construction Ltd | Construction | 2022 |
| Organic Remedies TCM Ltd | Other services | 2018 |
| Origin Wave Limited | Information & communication | 2024 |
| Orinoco Solutions Ltd | Administrative & support | 2008 |
| Outbox Europe Limited | Wholesale & retail | 2020 |
| Outdoor Options Limited | Administrative & support | 2000 |
| Outshine Cleaning Services Limited | Administrative & support | 2018 |
| Outsource Clothing Ltd | Wholesale & retail | 2024 |
| Outsource Office Limited | Real estate | 2001 |
| Oval Business Solutions Limited | Extraterritorial | 2009 |
| Oval Systems Limited | Administrative & support | 2003 |
| OWG Plumbing & Heating Ltd | Construction | 2021 |
| Oxford Education Analytics Ltd | Information & communication | 2019 |
| Oxpoint Limited | Construction | 2007 |
| Oxwood Ltd | Professional & scientific | 2016 |
| Oyyl Ltd | Wholesale & retail | 2020 |
| OZ Equine Ltd | Agriculture & fishing | 2020 |