| North Herts Surfacing Limited | Construction | 1989 |
| North Norfolk HH Limited | Other services | 2023 |
| Novanthro Ltd | Professional & scientific | 2026 |
| Novelcit Solutions Ltd | Wholesale & retail | 2020 |
| Novisim Limited | Information & communication | 2004 |
| NP Mortgages Limited | Financial services | 2018 |
| NP Systems Engineering Ltd | Professional & scientific | 2023 |
| Nreliability Ltd | Information & communication | 2022 |
| Nstoner Zervices Limited | Construction | 2025 |
| Nucleon Limited | Professional & scientific | 2020 |
| Nucleus Management Holdings Limited | Professional & scientific | 2002 |
| Nucleus Management Limited | Other services | 1988 |
| Nucore Ltd | Professional & scientific | 2025 |
| Nude Design Limited | Professional & scientific | 2003 |
| Nueronetics Ltd | Professional & scientific | 2025 |
| Nurtured Revolution Ltd | Professional & scientific | 2018 |
| Nuthampstead Shooting Ground LLP | | 2021 |
| Nutwood Developments Limited | Construction | 1995 |
| O B Inns LLP | | 2023 |
| O.C. Electrical Ltd | Construction | 2008 |
| Oakbridge Project Consultants Ltd. | Construction | 2021 |
| Oakridge Farms Limited | Agriculture & fishing | 1958 |
| Oakwhite Contracts Ltd | Construction | 2009 |
| Oakwood Business Systems Limited | Information & communication | 1986 |
| Oasis Promotions and Print Ltd | Manufacturing | 2016 |
| Oaudio Limited | Wholesale & retail | 2021 |
| OCD Cars Limited | Wholesale & retail | 2022 |
| OCD Parts Ltd | Manufacturing | 2022 |
| Ocean Frame Properties Ltd | Households | 2025 |
| ODT Safety Solutions Ltd | Professional & scientific | 2026 |
| OEM Parts World Ltd | Wholesale & retail | 2022 |
| OF Digital Ltd | Information & communication | 2024 |
| Offkut Studio Ltd | Manufacturing | 2022 |
| OFL Fashions Ltd | Wholesale & retail | 2021 |
| Old Coachworks Limited | Construction | 2025 |
| Oldham Wednesday Limited | Administrative & support | 2020 |
| OLI International Ltd | Information & communication | 2010 |
| Oliver Fraser Consultancy Ltd | Professional & scientific | 2026 |
| Oliver's Wills Limited | Professional & scientific | 2008 |
| Olorire Media Ltd | Wholesale & retail | 2025 |
| OMK Holdings Ltd | Financial services | 2025 |
| ON Court Performance Limited | Professional & scientific | 2010 |
| ON Point Carpentry and Joinery Ltd | Construction | 2018 |
| One OFF Oak Limited | Manufacturing | 2012 |
| Onetruegrit Ltd | Administrative & support | 2017 |
| Open Intel Solutions Ltd | Information & communication | 2024 |
| Orange-Tech Ltd | Professional & scientific | 2026 |
| Orchid Information Systems Limited | Information & communication | 1996 |
| Orcom Solutions Limited | Information & communication | 2012 |
| Ordapad Limited | Information & communication | 2019 |
| Origin Amenity Limited | Agriculture & fishing | 2011 |
| Origin Amenity Solutions Limited | Wholesale & retail | 1919 |
| Origin Northern Ireland Limited | Manufacturing | 1985 |
| Origin Suregreen Limited | Wholesale & retail | 2023 |
| Origin UK Operations Limited | Manufacturing | 1990 |
| Orion Wildlife Ltd | Education | 2005 |
| Oryza Bioinformatics Ltd | Professional & scientific | 2017 |
| Osmo Group Limited | Wholesale & retail | 2025 |
| Ossidense Ltd | Health & social care | 2025 |
| Ostrea Biotech Limited | Professional & scientific | 2024 |
| Outside of the Box CIC | Education | 2024 |
| Over the Rainbow Parties Limited | Health & social care | 2018 |
| OVP Empreendimentos Limited | Professional & scientific | 2014 |
| Ovrload Limited | Arts & recreation | 2024 |
| Owzat Limited | Extraterritorial | 1996 |
| Oyinayo Foods & Events Limited | Wholesale & retail | 2025 |
| OZ Espresso Machines Ltd. | Wholesale & retail | 2011 |
| P & a Insolvency Services Limited | Administrative & support | 2005 |
| P D Lidge Ltd | Construction | 2021 |
| P L Cooper & Sons Limited | Construction | 2003 |
| P. & H. Publications Limited | Real estate | 1982 |
| P.& V.Bishop Investments Limited | Real estate | 1960 |
| P.E.G. Williams Ltd | Financial services | 2024 |
| P.J. Robinson Electrical Contractors Limited | Construction | 1995 |
| P.S.R. Fashions Limited | Wholesale & retail | 1977 |
| P2 Performance Squared Limited | Professional & scientific | 2009 |
| PAD Digital and AI Services UK Limited | Information & communication | 2019 |
| Padco Developments Ltd | Households | 2017 |
| Padfield Estates LLP | | 2012 |
| Pall Mall Investment Capital Limited | Financial services | 2019 |
| PAN Developments Limited | Construction | 2004 |
| Pandiavalentin Ltd | Manufacturing | 2024 |
| Pandion Consultants Limited | Information & communication | 2020 |
| Papplewick Energy LLP | | 2013 |
| Papworth Solutions Limited | Administrative & support | 2023 |
| Parachutiste Limited | Professional & scientific | 1999 |
| Paranannies Limited | Financial services | 2020 |
| Parenting Profiles Associates Limited | Health & social care | 1999 |
| Parker Farms Holdings Limited | Agriculture & fishing | 2024 |
| Parkes Medical Limited | Health & social care | 2010 |
| Parnutt Foods Limited | Extraterritorial | 1985 |
| Parrmolloy Mechanical Ltd | Construction | 2025 |
| Particle Works Limited | Manufacturing | 2022 |
| Pascuzzi Brothers Limited | Extraterritorial | 2015 |
| Pathway Space Ltd | Professional & scientific | 2021 |
| Patrick Duffin Ltd | Arts & recreation | 2019 |
| Paul and Page 19 Royston Road Ltd | Real estate | 2018 |
| Paul and Page Limited | Construction | 2015 |
| Paul and Page Properties Limited | Real estate | 2014 |
| Paul Bowskill Ltd | Construction | 2021 |