| OK Hair Stylists Ltd | Other services | 2010 |
| OK Properties Cambridge Limited | Real estate | 2022 |
| Okava Ltd | Information & communication | 2022 |
| Okclick Tech Limited | Wholesale & retail | 2024 |
| Okowa International Law Ltd | Professional & scientific | 2021 |
| Okrs London Limited | Real estate | 2019 |
| Oktem Biopharma Consulting Ltd | Professional & scientific | 2019 |
| Okuni Industry&Trade (Dalian) Co., Ltd. | Extraterritorial | 2015 |
| Olan Research Limited | Education | 2007 |
| Oland Home Limited | Real estate | 2019 |
| OLARUVALENTIN1986 Ltd | Manufacturing | 2026 |
| Olaya Essentials Ltd | Wholesale & retail | 2024 |
| Old Car Ltd | Manufacturing | 2025 |
| Old Forge Apiaries Ltd | Agriculture & fishing | 2018 |
| Old Forge Electrical and Maintenance Ltd | Construction | 2025 |
| Old Granary Developments Limited | Construction | 2006 |
| Old Mill Furniture Limited | Transport & logistics | 2001 |
| Old Orchard Over LLP | | 2014 |
| Old School Court (Great Shelford) Management Company Limited | Households | 2011 |
| Old School Court Residents Management Company Limited | Households | 2006 |
| Old School Eden Street Residents Company Limited(The) | Real estate | 1982 |
| Old Silloth Farm Limited | Agriculture & fishing | 2020 |
| Old ST Paul's (Cambridge) Freehold Limited | Households | 2005 |
| Old ST. Paul's Residents Association Limited | Households | 1985 |
| Old Station Place Chatteris Management Limited | Households | 2006 |
| Old Stone Developments Ltd | Construction | 2019 |
| Old Stratford (Milton Keynes) Management Company Limited | Households | 2005 |
| Olek Electrik Ltd | Manufacturing | 2017 |
| OLI Kelly Ltd | Information & communication | 2025 |
| Oligotune Ltd | Professional & scientific | 2023 |
| Olimak Ltd | Health & social care | 2022 |
| Oliva Hospitality Limited | Hospitality | 2023 |
| Olivaliu Ltd | Manufacturing | 2026 |
| Olive Aesthetics Ltd | Other services | 2024 |
| Olive Engineering Ltd | Professional & scientific | 2020 |
| Olive's Coffee Shop Limited | Hospitality | 2021 |
| Oliver James Properties Limited | Real estate | 2023 |
| Olivia Design Ltd | Extraterritorial | 2016 |
| Olivmia Limited | Professional & scientific | 2024 |
| Ollie's Disco Wagon Limited | Transport & logistics | 2024 |
| OLO LUC Ltd | Real estate | 2025 |
| Olskam Ltd. | Wholesale & retail | 2017 |
| Olteanclaudiu Composites Ltd | Manufacturing | 2022 |
| Oltera Ltd | Professional & scientific | 2024 |
| Olympus Tiles Limited | Wholesale & retail | 1985 |
| OM Cast Consultancy Ltd | Information & communication | 2026 |
| Omanes Care Limited | Health & social care | 2013 |
| Omarb Limited | Health & social care | 2012 |
| Omed Health Limited | Health & social care | 2022 |
| Omega 4 London (UK) Ltd | Real estate | 2022 |
| Omer Maris Ltd | Information & communication | 2026 |
| Omia Retail Ltd | Wholesale & retail | 2021 |
| Omicaxis Biotechnology Consulting Ltd | Professional & scientific | 2025 |
| Omicron Consultancy Limited | Administrative & support | 2015 |
| Omicron Enterprise Holdings Limited | Financial services | 2021 |
| Omid Genetics Limited | Health & social care | 2020 |
| Ominira Property Limited | Real estate | 2023 |
| Omni Data Quality Ltd | Professional & scientific | 2015 |
| Omnibuds Ltd | Manufacturing | 2024 |
| Omnigen Biodata Ltd | Professional & scientific | 2017 |
| Omnimercado Limited | Wholesale & retail | 2024 |
| Omnino Limited | Administrative & support | 2010 |
| Omnisense Limited | Information & communication | 2008 |
| Omoni Limited | Health & social care | 2025 |
| OMS Range Ltd | Wholesale & retail | 2021 |
| ON Fleek Events Ltd | Arts & recreation | 2025 |
| ON NAP Consultancy Limited | Professional & scientific | 2024 |
| ON Point Basketball Limited | Education | 2026 |
| ON Roofing Ltd | Construction | 2025 |
| Oncampus Aston Limited | Education | 2020 |
| Oncampus Coventry Limited | Education | 2007 |
| Oncampus Hull Ltd | Education | 2009 |
| Oncampus London Limited | Education | 2009 |
| Oncampus Loughborough Limited | Education | 2022 |
| Oncampus Lsbu Limited | Education | 2007 |
| Oncampus Northumbria Limited | Education | 2026 |
| Oncampus Reading Limited | Education | 2016 |
| Oncampus Southampton Limited | Education | 2021 |
| Oncampus Sunderland Limited | Education | 2009 |
| Oncampus UK North Limited | Education | 2007 |
| Oncoelectronics Ltd | Manufacturing | 2025 |
| Oncoweb Limited | Extraterritorial | 1996 |
| Ondemand Limited | Transport & logistics | 2013 |
| One Cart Limited | Wholesale & retail | 2024 |
| One Day Roastery Ltd | Hospitality | 2025 |
| One Glove Films Ltd | Information & communication | 2017 |
| One Lead Two Minds Ltd | Other services | 2026 |
| One Leaf Green Educational Psychology Ltd | Education | 2025 |
| One Nucleus Limited | Other services | 2000 |
| One Part Love Limited | Manufacturing | 2017 |
| One Place TCG Ltd | Wholesale & retail | 2024 |
| One Planet Capital Limited | Financial services | 2020 |
| One Stop Recruiters Ltd | Administrative & support | 2025 |
| One the Elephant Limited | Extraterritorial | 2013 |
| One Two Enterprises Ltd | Wholesale & retail | 2021 |
| One Zero Accountancy Limited | Professional & scientific | 2006 |
| One Zero Outsourcing Ltd | Administrative & support | 2020 |
| One Zero Paye Ltd | Administrative & support | 2018 |
| Onebody Physiotherapy Ltd | Health & social care | 2012 |
| Onebound Limited | Wholesale & retail | 2024 |