| Nomandia Ltd | Wholesale & retail | 2018 |
| Noosasoona Ltd | Professional & scientific | 2012 |
| Norris Inspection Limited | Professional & scientific | 2022 |
| North 51 Mining Limited | Professional & scientific | 2020 |
| Northstar Strategy Ltd | Information & communication | 2024 |
| Norton Alexander Ltd | Education | 2019 |
| NOT IF When Ltd | Administrative & support | 2025 |
| Note Audio Ltd | Wholesale & retail | 2010 |
| Nott Copy Ltd | Information & communication | 2024 |
| Nova Luna Ltd | Professional & scientific | 2025 |
| Nowinsync Limited | Information & communication | 2023 |
| NP Electrics & Testing Ltd | Construction | 2014 |
| NSI Developments Limited | Construction | 2024 |
| NSI Properties Limited | Real estate | 2012 |
| NU - Logic Limited | Information & communication | 1999 |
| Null SUM Ltd | Information & communication | 2022 |
| NUT Hazel Cross Holdings Ltd | Financial services | 2022 |
| NXT Systems Ltd | Information & communication | 2024 |
| O Dwyers Ltd | Real estate | 2026 |
| O&A Contracts Ltd | Public administration | 2020 |
| O&G Energy Consulting Limited | Professional & scientific | 2024 |
| O.N. Production Services Ltd | Information & communication | 2022 |
| Oakleaf Development Ltd | Construction | 2020 |
| Oasis Landscape and Design Limited | Administrative & support | 2018 |
| Obitsol Ltd | Information & communication | 2017 |
| Ocean 56 Restaurants and Catering Ltd | Hospitality | 2025 |
| Octagon Automotive Ltd | Information & communication | 2020 |
| OCY Service Ltd | Information & communication | 2016 |
| OH Lucky Ltd | Wholesale & retail | 2025 |
| OKL Associates Ltd | Professional & scientific | 2018 |
| Old Boys of Luton 2015 Limited | Hospitality | 2015 |
| Oliver Burns LLP | | 2013 |
| Oliver Burns Studio Ltd | Professional & scientific | 2023 |
| Oliver Geraghty Rigging Ltd | Information & communication | 2022 |
| Oliver Myles Events Limited | Administrative & support | 2011 |
| OLJ Trading Limited | Wholesale & retail | 2017 |
| Olney Mens Shed Community Interest Company | Health & social care | 2025 |
| Olsson-Francis Ltd | Professional & scientific | 2025 |
| Omega Solutions Limited | Information & communication | 2023 |
| Omegamami Ltd | Hospitality | 2024 |
| ON AV Limited | Information & communication | 2014 |
| ON IT Services Limited | Information & communication | 2003 |
| ON the Limit Gym Ltd | Arts & recreation | 2010 |
| On-Site Promotions Limited | Professional & scientific | 2011 |
| One Eyed Craftsman Ltd | Information & communication | 2015 |
| One Two Three Hammersmith Grove Limited | Professional & scientific | 1990 |
| Onefivethree Property Ltd | Real estate | 2026 |
| Onefivethree Retail Ltd | Wholesale & retail | 2026 |
| Oneoneone Solutions Ltd | Information & communication | 2024 |
| Ontel Services Ltd | Information & communication | 2015 |
| Opal Studio Ltd | Information & communication | 2019 |
| Opscraft Limited | Information & communication | 2024 |
| Opsec Consulting Limited | Information & communication | 2019 |
| Optical Fibre Infrastructure Limited | Information & communication | 2018 |
| Optimum Health and Lifestyle Limited | Extraterritorial | 2024 |
| Options Mail Order Software Limited | Information & communication | 1996 |
| Optisights Ltd | Information & communication | 2018 |
| Orange Turtle Consulting Ltd | Information & communication | 2022 |
| Orchard Gardens (Tring) Management Company Ltd | Households | 2012 |
| Orchard Holdco Limited | Financial services | 2024 |
| Orchard Independent Financial Advice Limited | Extraterritorial | 2001 |
| Orchards Longfield Road Residents Association Limited(The) | Households | 1976 |
| Orchestrate LAB Ltd | Information & communication | 2026 |
| Orchid Estate Agents (Boxmoor) Ltd | Real estate | 2018 |
| Orchid Estate Agents Limited | Real estate | 2006 |
| Orchid Land and New Homes Ltd | Real estate | 2018 |
| Orgtopia Limited | Professional & scientific | 2002 |
| Original Writer Limited | Professional & scientific | 2023 |
| Orint Developments & Support Services Ltd | Construction | 2019 |
| Orint Limited | Administrative & support | 2002 |
| Orliam Consulting Ltd | Administrative & support | 2016 |
| Orliam Sports Ltd | Wholesale & retail | 2024 |
| Orofena Films Limited | Information & communication | 2009 |
| Orthodontic House Ltd | Health & social care | 2025 |
| Osology Ltd | Information & communication | 2018 |
| Outline Press (Book Publishers) Limited | Information & communication | 1988 |
| Over the Rainbow CSF Ltd | Education | 2025 |
| Oversubscribed Ltd | Professional & scientific | 2020 |
| Overx Limited | Professional & scientific | 2017 |
| P a C Transport Ltd | Transport & logistics | 2014 |
| P Cook Electrical Ltd | Construction | 2016 |
| P Cook Limited | Construction | 2025 |
| P D L Builders Limited | Construction | 2002 |
| P Edgar Ltd | Professional & scientific | 2019 |
| P Marsden Ltd | Health & social care | 2022 |
| P&M Carpets Ltd | Wholesale & retail | 2024 |
| P.A.B. Fixings Limited | Construction | 2007 |
| Pablo Antiq Transport Ltd | Transport & logistics | 2017 |
| Pacificworks Limited | Other services | 2005 |
| Paddlefusion Limited | Arts & recreation | 2024 |
| Page Green Recruitment Ltd. | Administrative & support | 2011 |
| Painesend Farm LLP | | 2005 |
| Palmers Business Support Ltd | Professional & scientific | 2001 |
| Palmers Property Services Ltd | Real estate | 2021 |
| Parachute Professional Services Ltd | Administrative & support | 2013 |
| Parahealthcare Limited | Health & social care | 2023 |
| Parallax Digital Limited | Professional & scientific | 2025 |
| Park Inn 888 | Arts & recreation | 2026 |
| Park Lane Computing Ltd | Information & communication | 2022 |
| Paul Bellfield Ltd | Construction | 2024 |