| Northdale Asset Management Limited | Professional & scientific | 2010 |
| Northdale Real Estate Partners Limited | Professional & scientific | 2015 |
| Northstar Value Advisory Limited | Professional & scientific | 2026 |
| NOT for Profit Services Limited | Administrative & support | 2025 |
| Notable Developments Limited | Information & communication | 2004 |
| Nouryzal London Limited | Health & social care | 2017 |
| Nova Elevators UK Ltd | Extraterritorial | 2022 |
| Nova London International Limited | Real estate | 2026 |
| Nova Medical Supplies Ltd | Wholesale & retail | 2026 |
| Nova Wedding Planning Ltd | Hospitality | 2024 |
| Novaonemanagement Ltd | Professional & scientific | 2026 |
| NP Oncology Ltd | Health & social care | 2019 |
| NPM Packaging Consultants Ltd | Administrative & support | 2016 |
| NR Construction Ltd | Construction | 2024 |
| NR Diagnostics Ltd | Health & social care | 2017 |
| NSJ Joinery Limited | Construction | 2016 |
| Nubax Consultants Ltd | Professional & scientific | 2025 |
| Nubax Limited | Professional & scientific | 2004 |
| Nuco Creative Limited | Professional & scientific | 2013 |
| Nudge Investments Ltd | Administrative & support | 2011 |
| Nuff Limited | Administrative & support | 2022 |
| Nutshell Electrics Limited | Construction | 2024 |
| NY Trading Limited | Wholesale & retail | 2021 |
| Nyra Services Limited | Information & communication | 2021 |
| O J Associates Limited | Real estate | 2019 |
| O W a Solutions Limited | Information & communication | 2016 |
| O'Connell and Patel Medical Ltd | Health & social care | 2022 |
| Oak & Crust Ltd | Hospitality | 2026 |
| Oak Hillside Developments Limited | Real estate | 2019 |
| Oak House Interiors Ltd | Wholesale & retail | 2003 |
| Oak House Retail Ltd | Wholesale & retail | 2019 |
| Oak Tree Barn Weddings Limited | Real estate | 2020 |
| Oak Tree Family Investment Company Limited | Financial services | 2025 |
| Oak Tree Farm Pet Foods Limited | Manufacturing | 2018 |
| Oak Tree Pottery Ltd | Manufacturing | 2026 |
| Oakdene Financial Services Limited | Financial services | 2017 |
| Oakfield Talent Development Limited | Professional & scientific | 2021 |
| Oakfield TQ Limited | Other services | 2013 |
| Oakhurst Property Solutions Limited | Construction | 2014 |
| Oaklake Limited | Construction | 2023 |
| Oakleigh Search Partners Limited | Other services | 2019 |
| Oakley Van Conversions Ltd | Wholesale & retail | 2023 |
| Oaks Business Consulting Limited | Education | 2011 |
| Oakwood Conversions Limited | Construction | 2023 |
| Oast Bridal Limited | Other services | 2026 |
| Oast House Place Management Company Limited | Households | 2026 |
| OB Media Ltd | Information & communication | 2009 |
| Obsolete Film Production Ltd | Information & communication | 2026 |
| OC & BC Residents Association Limited | Households | 2025 |
| Ocean Heights Freehold Limited | Real estate | 2023 |
| Oceanum NO2 Limited | Professional & scientific | 2011 |
| Ocwalrond Ltd | Construction | 2015 |
| ODT Management Services Limited | Professional & scientific | 2003 |
| OFF Piste Wealth Limited | Professional & scientific | 2021 |
| Ohmwork Learning Ltd | Information & communication | 2026 |
| OI2 Ltd | Health & social care | 2023 |
| OIS Media Limited | Information & communication | 2000 |
| OJB Resourcing Limited | Administrative & support | 2020 |
| OJC Distribution Limited | Wholesale & retail | 2026 |
| Okapi House Ltd | Professional & scientific | 2026 |
| Olaoju Consulting Ltd | Administrative & support | 2016 |
| Old Auction House Ltd | Hospitality | 2021 |
| Old Hadlow Road Paddock Limited | Professional & scientific | 2010 |
| Old HAY Historic Aircraft Limited | Transport & logistics | 2010 |
| Old Market (Marden) Residents Management Company Limited | Real estate | 2009 |
| Old Market (Marden) Road Management Limited | Real estate | 2009 |
| Old Park Classics Limited | Wholesale & retail | 2020 |
| Old Station Garage Ltd | Wholesale & retail | 2019 |
| Olive Consultants Ltd | Professional & scientific | 2023 |
| Oliver Mitchell Estates Ltd | Real estate | 2024 |
| Olivia Jacob Ltd | Real estate | 2017 |
| OLR Contracts Limited | Construction | 2018 |
| Olsen Radiology Ltd | Health & social care | 2020 |
| Olyvon ARK Ltd | Real estate | 2026 |
| Olyvon Films Ltd | Information & communication | 2023 |
| Olyvon Ltd | Information & communication | 2011 |
| OM Anandashram Ltd | Real estate | 2022 |
| Omega Business Finance Ltd | Administrative & support | 2010 |
| OMG Incorporated Limited | Professional & scientific | 2021 |
| Ommd Limited | Professional & scientific | 2016 |
| Omnivisto Limited | Information & communication | 2020 |
| Omolara Property Limited | Real estate | 2022 |
| OMS Support Ltd | Administrative & support | 2021 |
| ON the Right Track | Professional & scientific | 2015 |
| Oncall Personnel Limited | Administrative & support | 1998 |
| One Event Media Group Limited | Professional & scientific | 2018 |
| One Grey Dog Holdings Limited | Arts & recreation | 2020 |
| One OFF Event Ltd | Other services | 2019 |
| One Seven Four Hair & Beauty Ltd | Other services | 2021 |
| Onedee Thai Ltd | Other services | 2025 |
| Online Elevation Ltd | Professional & scientific | 2021 |
| Ontrack Commercial Fencing Limited | Construction | 2026 |
| Onxe Limited | Information & communication | 2024 |
| Onyx Executive Ltd. | Other services | 2019 |
| Oojamaflips Ltd | Real estate | 2021 |
| OPENDOOR54 Ltd | Real estate | 2020 |
| Ophthalmic Supplies & Solutions Ltd | Construction | 2019 |
| Oppa Limited | Professional & scientific | 2020 |
| Optical Data Technology Ltd | Information & communication | 2010 |
| Optiflex Utilities Limited | Construction | 2025 |