| Oakengates Carpet Company Limited | Real estate | 1965 |
| Oakengates Foundry Limited | | 1986 |
| Oakengates Funeral Services Ltd | Professional & scientific | 2013 |
| Oakfield Integrated Property Services Limited | Real estate | 2024 |
| Oaking Developments 3 Limited | Construction | 2016 |
| Oaklands Farm Eggs Limited | Agriculture & fishing | 1962 |
| Oaklands Minerals Limited | Professional & scientific | 1964 |
| Oakley Business Services Limited | Real estate | 1998 |
| Oakshield Real Estate Ltd | Real estate | 2025 |
| Oakvalley Joists and Roofs Limited | Construction | 2021 |
| Oany International Ltd | Wholesale & retail | 2025 |
| Oasis Group Holdings Ltd | Real estate | 2025 |
| Oasis VM Ltd | Professional & scientific | 2024 |
| OB 3D Prints Limited | Manufacturing | 2023 |
| Ob-Servers Limited | Information & communication | 2002 |
| Obasut Ltd | Other services | 2018 |
| Obsidian Project Solutions Ltd | Professional & scientific | 2025 |
| Obuz Consulting Limited | Professional & scientific | 2026 |
| OCD Automotive Ltd | Wholesale & retail | 2020 |
| Oceala Limited | Water & waste | 1982 |
| Octane Advisory Ltd | Professional & scientific | 2026 |
| Octavia Multimedia Group Limited | Professional & scientific | 2016 |
| Octavia Music Magazine Ltd | Professional & scientific | 2016 |
| Octosim Ltd | Other services | 2019 |
| OD Fastfood Limited | Hospitality | 2024 |
| ODD Orange Limited | Manufacturing | 2021 |
| Oddwheels R US Limited | Extraterritorial | 2019 |
| Odin Projects Limited | Professional & scientific | 2017 |
| Odlik Ltd | Wholesale & retail | 2019 |
| OEA Trading Limited | Financial services | 2013 |
| OFD Racing Ltd | Arts & recreation | 2025 |
| Offpiste Marketing Limited | Administrative & support | 2014 |
| Ofsima Ltd | Other services | 2019 |
| Ogoh Multi Service Ltd | Administrative & support | 2026 |
| OH SO Pretty Ltd | Wholesale & retail | 2025 |
| Ohana Bookkeeping Ltd | Professional & scientific | 2025 |
| Ohhh Limited | Financial services | 2025 |
| OHU Commercials Ltd | Wholesale & retail | 2026 |
| OIL Genie Automotive Ltd | Financial services | 2022 |
| Oishii Limited | Wholesale & retail | 2024 |
| OJ Property Solutions Limited | Administrative & support | 2014 |
| OJP Applications Ltd | Information & communication | 2020 |
| OJR Investments Ltd | Real estate | 2018 |
| OJS (UK) Limited | Wholesale & retail | 2011 |
| OJS Properties Limited | Financial services | 2020 |
| Okko Aesthetics Limited | Other services | 2019 |
| Oklu Autos Ltd | Wholesale & retail | 2025 |
| Oklu Properties Ltd | Real estate | 2025 |
| Olbias Ltd | Other services | 2024 |
| Old Hall School Trust (Wellington) Limited(The) | Education | 1959 |
| Old Hall SPV Limited | Real estate | 2021 |
| Old Park Services Limited | Administrative & support | 2000 |
| Old School Entertainment Ltd | Arts & recreation | 2018 |
| Old Town 2023 Limited | Hospitality | 2023 |
| OLD-PRINTS4ALL.COM Limited | Extraterritorial | 2025 |
| Oldfield Software Solutions Ltd | Professional & scientific | 2026 |
| Olefic Ltd | Other services | 2020 |
| Olian Ltd | Construction | 2015 |
| Olimkat Ltd | Other services | 2019 |
| Olirb Ltd | Other services | 2025 |
| Olitek Limited | Construction | 2011 |
| Olivebright Ltd | Professional & scientific | 2026 |
| Oliver-Kyriacou Solutions Limited | Professional & scientific | 2023 |
| Olivia Parsons Ltd | Real estate | 2025 |
| Ollie K Limited | Health & social care | 2016 |
| Omar SL Ltd | Wholesale & retail | 2025 |
| Omatomzy Consulting Limited | Health & social care | 2026 |
| Omega Electrical Installations Ltd | Construction | 2018 |
| Omicron Designs Ltd | Professional & scientific | 2019 |
| Omni-Group UK Holdings Ltd | Financial services | 2025 |
| Omni-Racing Limited | Manufacturing | 2025 |
| Omniforce Supplements Ltd | Wholesale & retail | 2025 |
| Omnigreen Ltd | Professional & scientific | 2014 |
| Omnii Management Services Limited | Professional & scientific | 2023 |
| Omnisync Grid AI Ltd | Information & communication | 2026 |
| Omnital Ltd | Other services | 2019 |
| Omnitech Ltd | Manufacturing | 2022 |
| Omonia Ltd | Other services | 2020 |
| Omsai Softech Limited | Information & communication | 2013 |
| OMW Property Limited | Real estate | 2024 |
| ON Cloud Nine Ltd | Administrative & support | 2019 |
| ON Grid Ltd | Construction | 2025 |
| One Call Traffic Management Limited | Other services | 2019 |
| One Heat Loss Ltd | Professional & scientific | 2023 |
| One Source Construction Services Ltd | Construction | 2020 |
| One Source Powered Access Ltd | Administrative & support | 2026 |
| One Stop Windows Limited | Manufacturing | 2021 |
| Oneil’S Ltd | Transport & logistics | 2025 |
| Onis Leisure Ltd | Hospitality | 2024 |
| Online Drainage Solutions Limited | Construction | 2023 |
| Online Transport Services Limited | Transport & logistics | 2025 |
| ONLY4UPRO Ltd | Wholesale & retail | 2020 |
| Onur AB Holdings Limited | Wholesale & retail | 2024 |
| Onymar Ltd | Other services | 2020 |
| Onyx Alliance Limited | Wholesale & retail | 2022 |
| OPA Designs Ltd | Manufacturing | 2021 |
| Opek Property Limited | Real estate | 2023 |
| Open Cognitive Limited | Information & communication | 2016 |
| Openstem Ltd | Education | 2023 |
| Opihc Limited | Health & social care | 2014 |