| Oldfield Mews Management Co. Limited | Households | 1977 |
| Oldfield Property Investments Limited | Real estate | 2022 |
| Oldham Seals Group Limited | Financial services | 2019 |
| Oldham Seals Limited | Manufacturing | 1964 |
| OLE Sports Ltd | Arts & recreation | 2016 |
| Olinda Community Gardens C.I.C. | Arts & recreation | 2023 |
| Olink Proteomics Limited | Professional & scientific | 2015 |
| Olive Casting Ltd | Administrative & support | 2022 |
| Olive Property Care Services Limited | Real estate | 2022 |
| Oliver Crispin Robotics Limited | Manufacturing | 1997 |
| Oliver Hilton Consulting Limited | Professional & scientific | 2025 |
| Oliver James Vanguard Ltd | Administrative & support | 2026 |
| Oliver Legal Consulting Limited | Professional & scientific | 2023 |
| Ollerbarrow Strategy Ltd | Professional & scientific | 2019 |
| Ollerset View Limited | Households | 2001 |
| Ollie James Ltd | Information & communication | 2017 |
| Olliver Photography Limited | Administrative & support | 2012 |
| OLM Cyber Ltd | Information & communication | 2024 |
| OLO DIN UK Ltd | Information & communication | 2026 |
| OLO Intelligence Limited | Professional & scientific | 2025 |
| Olympus Engineering (Holdings) Limited | Manufacturing | 2014 |
| Olympus Engineering Limited | Manufacturing | 2000 |
| Olympus Manufacturing Limited | Administrative & support | 2011 |
| Omaze UK Limited | Administrative & support | 2019 |
| Ombak Trading Ltd | Wholesale & retail | 2015 |
| OME Enterprises Limited | Administrative & support | 2012 |
| Omni Executive Search International Limited | Administrative & support | 2005 |
| Omni Executive Search Limited | Administrative & support | 2005 |
| Omni IT Limited | Extraterritorial | 2002 |
| Omni Logic (UK) Limited | Administrative & support | 2014 |
| Omni Resource Management Solutions Limited | Administrative & support | 1996 |
| Omni Search & Selection Limited | Administrative & support | 2005 |
| Omnia Corporate Limited | Financial services | 2020 |
| Omnipute Limited | Professional & scientific | 2024 |
| Omnni Ltd | Wholesale & retail | 2024 |
| Omtjs Ltd | Hospitality | 2025 |
| ON the Level PR Limited | Administrative & support | 2015 |
| ON Top of the World Travel Ltd | Other services | 2006 |
| ON Track Comms Limited | Construction | 2023 |
| ON Track Driving Limited | Administrative & support | 2016 |
| On-Point Connect UK Ltd | Administrative & support | 2017 |
| Oncap Consulting Ltd | Transport & logistics | 2020 |
| Oncology Consults UK Ltd | Health & social care | 2011 |
| Ondeo Industrial Solutions UK Limited | Financial services | 2001 |
| One Automation Systems Ltd | Information & communication | 2023 |
| One Circle of Friends Limited | Professional & scientific | 2024 |
| One Fine Day Design Limited | Professional & scientific | 2003 |
| One Global Limited | Professional & scientific | 2018 |
| One Hundred and TEN Percent Limited | Wholesale & retail | 2020 |
| One Iota Group Limited | Financial services | 2025 |
| One Iota Limited | Information & communication | 2010 |
| One More WHY Limited | Professional & scientific | 2026 |
| One Solution Consultancy Limited | Information & communication | 2025 |
| One Stop Property Partners Ltd | Real estate | 2022 |
| Onedigital Consultancy Limited | Professional & scientific | 2020 |
| Onemore Holdings Ltd | Financial services | 2025 |
| Onix Holdings Limited | Professional & scientific | 1997 |
| Online Endeavours Ltd. | Other services | 2015 |
| Online Jewel Company Ltd | Wholesale & retail | 2020 |
| ONY Legal Services Limited | Professional & scientific | 2020 |
| ONZ Ltd | Real estate | 2021 |
| Ooolala Ltd | Wholesale & retail | 2025 |
| OP Rebuild Limited | Arts & recreation | 2024 |
| Opaque Light Ltd | Professional & scientific | 2026 |
| Opel Consultancy Ltd | Professional & scientific | 2011 |
| Opel Valley Limited | Administrative & support | 2004 |
| Open Assembly UK Limited | Professional & scientific | 2022 |
| Open Home Property Ltd | Construction | 2018 |
| Open Trade Technology Ltd | Information & communication | 2023 |
| Open Trade Technology Services Ltd | Information & communication | 2024 |
| Opengame Payroll UK Limited | Information & communication | 2022 |
| Openrent Ltd | Information & communication | 2011 |
| Opensignal Limited | Information & communication | 2012 |
| Opensys Limited | Information & communication | 2026 |
| Opnw Ltd | Real estate | 2025 |
| Oppein Home UK Limited | Wholesale & retail | 2022 |
| Oppizio Limited | Administrative & support | 2022 |
| Opteralife Ltd | Wholesale & retail | 2026 |
| Opti-Pro Services Ltd | Professional & scientific | 2014 |
| Optical Centre Supplies Limited | Wholesale & retail | 2003 |
| Optical Revolution Group Limited | Financial services | 2023 |
| Optilogic UK Limited | Information & communication | 2024 |
| Optim Resource Ltd | Professional & scientific | 2021 |
| Optimal Engineering Solutions Limited | Professional & scientific | 2018 |
| Optimaria Ltd | Information & communication | 2015 |
| Optimise Clinic Ltd | Health & social care | 2024 |
| Optimum Business Services Limited | Construction | 1986 |
| Optimum Medcare Ltd | Health & social care | 2025 |
| Optimum Team Solutions (NW) Ltd | Administrative & support | 2013 |
| Optimus Connections Limited | Professional & scientific | 2022 |
| Optiplan Consulting Ltd | Professional & scientific | 2026 |
| Optitech Limited | Professional & scientific | 2024 |
| Optom-Invest Ltd | Wholesale & retail | 2025 |
| Optomac Limited | Other services | 2008 |
| Optylon UK Limited | Real estate | 2024 |
| Opulent Antiques Ltd | Extraterritorial | 2025 |
| Opulent Leisure Ltd | | 2010 |
| Opus Asset Management Ltd | Construction | 2018 |
| Opus Tiling Limited | Construction | 2025 |
| ORA Projects Ltd | Manufacturing | 2013 |