| Nimbus Trade Ltd | Wholesale & retail | 2025 |
| Nintronics Limited | Wholesale & retail | 2008 |
| NJG Building Solutions Ltd | Construction | 2022 |
| Nkls Investments Limited | Real estate | 2020 |
| Nkpc Limited | Professional & scientific | 2010 |
| Nobarriersradio Ltd | Information & communication | 2018 |
| Noble Deeds Ltd | Manufacturing | 2025 |
| Noir Atelier Limited | Wholesale & retail | 2024 |
| Nola's Cleaning Limited | Administrative & support | 2026 |
| Nomagugu Ltd | Health & social care | 2022 |
| North London & Herts Property Maintenance Ltd | Construction | 2019 |
| Norton Gym WGC Limited | Arts & recreation | 2015 |
| NOSA.EHIGIE5 Ltd | Information & communication | 2023 |
| Note Art Ltd | Wholesale & retail | 2024 |
| Novatuition Ltd | Education | 2020 |
| Novumgen Holding Limited | Manufacturing | 2024 |
| Nowak Carpenters Ltd | Construction | 2019 |
| NP Infotech Limited | Information & communication | 2025 |
| NTB Consulting Services Ltd | Information & communication | 2022 |
| Numskull Games Ltd | Information & communication | 2019 |
| Nuptse Limited | Information & communication | 2015 |
| Nurcan Kacak Limited | Wholesale & retail | 2024 |
| Nursery on the Green Limited | Education | 2003 |
| Nursery on the Hill (Enfield) Limited | Education | 2006 |
| Nurtured by Nature Early Years Group Ltd | Health & social care | 2017 |
| Nurturing Childcare Ltd | Education | 2003 |
| NYO Fashion Limited | Wholesale & retail | 2021 |
| O Solutions Limited | Professional & scientific | 2008 |
| O'Reilly Property Services Ltd | Real estate | 2021 |
| O.T. Catering Ltd | Hospitality | 2023 |
| O.T. Lettings Ltd | Real estate | 2023 |
| O2J2 Limited | Professional & scientific | 2018 |
| OA Recruitment Limited | Other services | 2023 |
| Oak Rise Property Services Ltd | Construction | 2025 |
| Oak Rocks Consulting Limited | Professional & scientific | 2016 |
| Oakcroft Investments Limited | Wholesale & retail | 2018 |
| Oaklands Management Services Limited | Administrative & support | 1998 |
| Oaks Court Properties Ltd | Real estate | 2021 |
| Oakwood Distribution Limited | Administrative & support | 2006 |
| Obconnect Limited | Information & communication | 2019 |
| Oceanic Living Care Trading AS Prosperity Housing Group/Hertfordshire Ltd | Health & social care | 2024 |
| Octagenix Europe Limited | Wholesale & retail | 2022 |
| Oculorum London Ltd | Wholesale & retail | 2021 |
| Odessa Consultancy Ltd | Information & communication | 2017 |
| Oemoe Ltd | Health & social care | 2014 |
| Office Ideas Limited | Real estate | 2000 |
| OH Sourcing Ltd | Administrative & support | 2019 |
| Old Welwyn Brewery Limited | Manufacturing | 2022 |
| Olga Clinic Limited | Health & social care | 2026 |
| Olifant Organics Limited | Agriculture & fishing | 2024 |
| Olympus Marketing Limited | Information & communication | 2021 |
| OM Paid Media Ltd | Professional & scientific | 2026 |
| Omer Faruk Ozturk Ltd | Wholesale & retail | 2024 |
| Omna Limited | Extraterritorial | 2011 |
| Omni Research Limited | Professional & scientific | 2013 |
| Omniac Pharmconsult Limited | Professional & scientific | 2014 |
| OMS Utilities Ltd | Construction | 2023 |
| ON the Ball Creative Ltd | Professional & scientific | 2019 |
| Once More With Data Ltd | Professional & scientific | 2022 |
| One 2 One Personal Communications Limited | Professional & scientific | 2001 |
| One Media Optical UK Ltd | Manufacturing | 2012 |
| One One SIX Cockfosters Management Company Ltd | Households | 2021 |
| Onehealthinvest Ltd | Health & social care | 2023 |
| Onshuise Limited | Real estate | 2018 |
| Opalrange Limited | Administrative & support | 2010 |
| Opalrange Recruitment Consultancy Ltd | Administrative & support | 2010 |
| Opmark Limited | Wholesale & retail | 2026 |
| Optima Retail Ltd | Information & communication | 2010 |
| Orange Slab Limited | Financial services | 2024 |
| Orasis Vision Limited | Health & social care | 2009 |
| Orbit Professional Services Limited | Information & communication | 2020 |
| Orchard Electrical Contractors Ltd | Construction | 2020 |
| Orchard House Day Nurseries & Pre-School Limited | Health & social care | 2022 |
| Orchard Mews Limited | Real estate | 2004 |
| Origin Alpha Studio Ltd | Information & communication | 2025 |
| Origin Co. and Ltd | Other services | 2019 |
| Orion Accounts Limited | Professional & scientific | 2017 |
| Orpington (Station Road) Limited | Administrative & support | 2006 |
| Orton Lifts & Escalators Ltd | Construction | 2004 |
| Osborne Appointments Limited | Extraterritorial | 2004 |
| Osborne Property Holdings Ltd | Real estate | 2018 |
| Outcome Activation Ltd | Information & communication | 2023 |
| Outdoor Playhouses Ltd | Wholesale & retail | 2024 |
| Outsourced Telecom Limited | Professional & scientific | 2011 |
| Outsourced Waste Limited | Professional & scientific | 2012 |
| Overland Automotive Ltd | Wholesale & retail | 2019 |
| Oxygen Trust | Agriculture & fishing | 2010 |
| P & ROY Limited | Real estate | 2022 |
| P a Gardens and Landscaping Ltd | Administrative & support | 2021 |
| P Tyler Associates Limited | Wholesale & retail | 1992 |
| P Vithlani Services Limited | Health & social care | 2019 |
| P-Code Ltd | Information & communication | 2015 |
| P.B. Donoghue (Haulage & Plant Hire) Limited | Administrative & support | 1974 |
| P.L.J.K. Builders Limited | | 1980 |
| PAC Electrics Ltd | Construction | 2016 |
| Pacy Services Ltd | Administrative & support | 2025 |
| Paddington Packet Boat Developments Limited | Construction | 2017 |
| Palomar Studios Limited | Professional & scientific | 2026 |
| Palomark Ltd | Professional & scientific | 2026 |
| Panasonic Heating & Ventilation Air-Conditioning UK Ltd | Energy | 1989 |