| Nurture House Montessori Ltd | Professional & scientific | 2018 |
| Nutrifind Limited | Hospitality | 2025 |
| NUTS4HONEY Ltd | Wholesale & retail | 2015 |
| NVM Consultancy Services Ltd | Information & communication | 2022 |
| Nvtreasures Ltd | Wholesale & retail | 2026 |
| Nwamaka and Co Limited Limited | Professional & scientific | 2019 |
| NXM Ltd | Wholesale & retail | 2021 |
| Nágyla’S Rodizio Limited | Hospitality | 2025 |
| O & P Property Services Limited | Real estate | 2004 |
| O L UK Construction Limited | Construction | 2020 |
| O&H Management Limited | Real estate | 2022 |
| O.K. Hospitality Services Limited | Hospitality | 2015 |
| Oak & ASH Productions Limited | Professional & scientific | 2015 |
| Oak ASH Green PUB Ltd | Hospitality | 2023 |
| Oak Construction Limited | Administrative & support | 2010 |
| Oak Landscapes Ltd | Administrative & support | 2022 |
| Oak Promise Limited | Administrative & support | 2017 |
| Oakbury Fine Jewellery & Watches Ltd | Wholesale & retail | 2026 |
| Oakbury Watch Co (Holdings) Ltd | Financial services | 2025 |
| Oakbury Watch Co Ltd | Wholesale & retail | 2025 |
| Oakley (SE) Limited | Construction | 2001 |
| Oakley Fitness (TWE) Ltd | Arts & recreation | 2020 |
| Oakley Fitness Limited | Arts & recreation | 2011 |
| Oakley Southern Limited | Real estate | 2003 |
| Oakley Tree Ltd | Agriculture & fishing | 2021 |
| Oakmead Finance Ltd | Financial services | 2015 |
| Oakmont Building Solutions Ltd | Construction | 2019 |
| Oakmont Construction Limited | Construction | 2004 |
| Oakmount Grace Properties Limited | Professional & scientific | 2014 |
| Oakridge Electrical Limited | Construction | 2015 |
| Oakway Electrical Limited | Construction | 2017 |
| Oana's Magic Ltd | Other services | 2024 |
| Oasis Physiology Ltd | Health & social care | 2024 |
| OB Consulting Services Ltd | Professional & scientific | 2025 |
| OBI & the Biscuit Limited | Hospitality | 2025 |
| Obimi Ltd | Health & social care | 2025 |
| Obimii CIC | Education | 2025 |
| Obsa 2024 Limited | Other services | 2024 |
| OC Consulting Engineers Limited | Professional & scientific | 2017 |
| Ocean Dragon Ltd | Hospitality | 2024 |
| Oceanways Technologies Ltd | Information & communication | 2021 |
| Ochs Trading Market Ltd | Wholesale & retail | 2019 |
| OCI Properties Ltd | Real estate | 2022 |
| Ocio Limited | Arts & recreation | 2026 |
| Octagon FM Ltd | Administrative & support | 2025 |
| October Estates Limited | Real estate | 1990 |
| Octopus Game Ltd | Information & communication | 2021 |
| Ocular Interface Limited | Professional & scientific | 2021 |
| Odinary Space Limited | Real estate | 2023 |
| Odyssey Midlands Limited | Manufacturing | 2021 |
| OFF Lead Ltd | Other services | 2021 |
| Offshore Design Concepts Ltd | Construction | 2012 |
| Oghenovo Ltd | Health & social care | 2014 |
| Ohuk Limited | Administrative & support | 2004 |
| OIC Solutions Ltd | Information & communication | 2013 |
| OJ GAS and Heating Ltd | Construction | 2023 |
| OJD Plasterworks Ltd | Construction | 2019 |
| OJO Recruits Ltd | Administrative & support | 2020 |
| Ojoventures Ltd | Wholesale & retail | 2025 |
| OJP Limited | Information & communication | 2004 |
| OK Investments Limited | Professional & scientific | 2019 |
| Okafor Consulting Ltd | Administrative & support | 2010 |
| Okéla Limited | Professional & scientific | 2017 |
| OLA Home Care Ltd | Health & social care | 2025 |
| Olas (Kent) Limited | Hospitality | 2024 |
| Olatunji Investments Limited | Real estate | 2015 |
| Olayemi Jewellery Ltd | Manufacturing | 2022 |
| Old Broad Street Accountants Limited | Other services | 2023 |
| Old Broad Street Consultants Limited | Professional & scientific | 2019 |
| Old Broad Street Limited | Other services | 2020 |
| Old Church Street Accountants Ltd | Other services | 1994 |
| Old Forge Mews Limited | Households | 2001 |
| Old Worlde School House Management Company Limited | Households | 2009 |
| Olenska Limited | Hospitality | 2011 |
| Olive Property Letting Limited | Real estate | 2015 |
| Olive Tree Building Services Limited | Construction | 2014 |
| Oliver Landon Orpington Ltd | Real estate | 2022 |
| Olivia James Vitality Ltd | Other services | 2025 |
| Ollie's Kebab (Lingfield) Ltd | Hospitality | 2023 |
| Ollie's Kebab YE & Fish Bar Ltd | Hospitality | 2019 |
| Olney Maintenance Ltd | Administrative & support | 2024 |
| Olsenberry Limited | Households | 1981 |
| Olson Digital Marketing Ltd | Information & communication | 2025 |
| OLY Consultancy Limited | Professional & scientific | 2015 |
| Olympius Developments Limited | Construction | 2017 |
| OM Hrishabh Limited | Real estate | 2025 |
| OMC (GB) Limited | Wholesale & retail | 2009 |
| Omescape VR Limited | Arts & recreation | 2025 |
| Omikron Limited | Hospitality | 2015 |
| Ominic Design Limited | Professional & scientific | 2001 |
| Omne Compliance Ltd | Administrative & support | 2025 |
| Omnimodus Ltd | Administrative & support | 2011 |
| Omniscient 360 Limited | Professional & scientific | 2015 |
| Omotenashi Limited | Professional & scientific | 2018 |
| Omtrex Ltd | Professional & scientific | 2022 |
| ON Message Communications Limited | Professional & scientific | 2010 |
| ONA Freehold Estates Limited | Households | 2018 |
| OND Last Limited | Health & social care | 2023 |
| One Call Cleaning and Gardening Ltd | Administrative & support | 2015 |
| One Central Avenue Welling Bexley Ltd | Real estate | 2026 |