| Nunnery Fisheries Ltd | Agriculture & fishing | 2021 |
| Nurse Care 24 Limited | Administrative & support | 2023 |
| Nusylinks Global Merchandize & Beauty PTY Training Ltd | Wholesale & retail | 2024 |
| NUT Tree Homes Limited | Construction | 2008 |
| Nutmeg Spa Limited | Other services | 2009 |
| Nutra AID Ltd. | Manufacturing | 2015 |
| Nutra Tea Ltd. | Manufacturing | 2016 |
| Nutravit Ltd. | Wholesale & retail | 2016 |
| NUU Transport Limited | Transport & logistics | 2017 |
| Nuvis London Limited | Wholesale & retail | 2020 |
| Nuxium Consultants Limited | Information & communication | 2014 |
| NV 247 Services Ltd | Transport & logistics | 2023 |
| NW Contractor Services Ltd. | Construction | 2012 |
| Nwjad Properties Ltd | Real estate | 2025 |
| Nxisearch Ltd | Administrative & support | 2026 |
| Nyala Holdings Ltd | Hospitality | 2018 |
| Nysa Projects Ltd | Arts & recreation | 2025 |
| O'Connell Consulting Ltd | Professional & scientific | 2016 |
| O'Kane Lavers Ltd | Wholesale & retail | 2019 |
| Oafa Distributor Ltd | Wholesale & retail | 2023 |
| Oak and Sycamore Management Company at Westley Green Limited | Households | 2023 |
| Oak Brook Manor Residents Management Company Limited | Households | 2025 |
| Oak Care Limited | Health & social care | 2001 |
| Oak Glen (Apartments) Management Company Limited | Households | 2007 |
| Oak Hill Joinery Limited | Manufacturing | 2011 |
| Oak Mill Apartments (Management Company) Limited | Real estate | 2014 |
| Oakcrest Homes Ltd | Construction | 2017 |
| Oakfields Park (Halstead) Management Company Limited | Households | 2022 |
| Oakhill Recruitment Limited | Administrative & support | 2016 |
| Oakhurst Flat Management Company Limited | Administrative & support | 2004 |
| Oakland (Southern) Property Developments Limited | Real estate | 2004 |
| Oakland Grange Residents Limited | Households | 2005 |
| Oaklands Animal Health Ltd | Manufacturing | 2016 |
| Oaklands Employee Ownership Trustees Limited | Other services | 2019 |
| Oaklands Landscaping Limited | Administrative & support | 1997 |
| Oaklands Livery Company Limited | Agriculture & fishing | 2013 |
| Oaklands Park Thundersley Management Company Limited | Households | 2016 |
| Oaklands Residents Management Company Limited | Households | 2022 |
| Oaklea Development Ltd | Construction | 2025 |
| Oakley Green South Management Company Limited | Households | 2024 |
| Oakmead Accounting Limited | Professional & scientific | 2014 |
| Oakwood Facilities Management Ltd | Households | 2024 |
| Oakwood Farm Eggs Ltd | Wholesale & retail | 2025 |
| Oberoi Episode Hotels Limited | Other services | 2016 |
| Obscenitus Ltd | Transport & logistics | 2025 |
| Obscura Search Ltd | Administrative & support | 2025 |
| Obshak Ltd | Construction | 2022 |
| Obylix Ltd | Transport & logistics | 2023 |
| Occupational Health Services Limited | Health & social care | 2008 |
| Ocean Outlet Ltd | Wholesale & retail | 2021 |
| Ocean Real Fish Limited | Hospitality | 2020 |
| Ocean Scene Films Limited | Information & communication | 2003 |
| Ocean Scene Properties Limited | Real estate | 2024 |
| Ocrow Estate Management Limited | Professional & scientific | 2018 |
| Octagon Engineering Ltd. | Professional & scientific | 2012 |
| Octave Recruitment Ltd | Administrative & support | 2021 |
| Ocular Thoughts Media UK Limited | Professional & scientific | 2020 |
| ODD Whim Management Company Limited | Households | 2005 |
| Odkw Ltd | Health & social care | 2020 |
| Office Initiative Limited | Administrative & support | 2006 |
| Officemate (UK) Limited | Wholesale & retail | 2003 |
| Officetechhub Limited | Information & communication | 2013 |
| Official Melanie Jane Ltd | Other services | 2025 |
| Offprints Ltd | Information & communication | 2020 |
| Offshore Renewable Energy Limited | Professional & scientific | 2019 |
| Ohams Property Limited | Real estate | 2020 |
| Ohana Beauty Ltd | Other services | 2025 |
| Ohm-Elec Ltd | Construction | 2017 |
| Ohms and Watts (Services) Ltd | Construction | 2009 |
| OJ Abodes Ltd | Real estate | 2022 |
| OJM Maintenance Ltd | Administrative & support | 2026 |
| OJS Inventory Limited | Wholesale & retail | 2023 |
| Okay Box Services Limited | Education | 2026 |
| Oktober Zambia (Import & Export) Ltd. | Wholesale & retail | 2009 |
| OLA Cleaning Services Limited | Administrative & support | 2024 |
| OLA Chica Limited | Wholesale & retail | 2023 |
| Olaer Group Limited | Professional & scientific | 2005 |
| Olaer Holdings Limited | Professional & scientific | 2005 |
| Olaer International Limited | Professional & scientific | 2007 |
| Olaer Limited | Professional & scientific | 2005 |
| Old English Bricklayer Construction Ltd | Construction | 2025 |
| Old Forest Road (Winnersh) Management Company Limited | Households | 2019 |
| Old Hempstead House Management Limited | Real estate | 2015 |
| Old London Road Pre-School | Education | 2010 |
| Old Macdonald's Day Nursery Ltd | Education | 2001 |
| Old School Gardens Management Company Limited | Households | 2005 |
| Old Tom's Bar Bistro Limited | Wholesale & retail | 2012 |
| Old Town Brickwork Limited | Construction | 2026 |
| Old Town Clothing Ltd | Wholesale & retail | 2025 |
| Old Town Property Management Company Limited | Households | 2017 |
| Old Town Queensway Limited | Extraterritorial | 2014 |
| Old Town Records Ltd | Wholesale & retail | 2025 |
| Old Town Support Service Limited | Health & social care | 2022 |
| Olem Engineering Ltd | Professional & scientific | 2019 |
| Olena Engineering Services Ltd | Construction | 2016 |
| Oleum Ventures Limited | Professional & scientific | 2016 |
| Olive 24 Ltd | Hospitality | 2024 |
| Olive Hemel Limited | Hospitality | 2019 |
| Olive Supported Accommodation Ltd | Real estate | 2025 |
| Oliver Famili Estate Ltd | Financial services | 2024 |