| OLADOTUNDADA@247HEALTHCARE Ltd | Professional & scientific | 2025 |
| OLADOTUNSAM247HEALTHCARE Ltd | Health & social care | 2025 |
| Olaore Aesthetics Ltd | Health & social care | 2026 |
| Olaore Dental Limited | Health & social care | 2022 |
| Old Market (EN) Ltd | Hospitality | 2017 |
| Old Regent Group Ltd | Wholesale & retail | 2025 |
| Old Times 2025 Ltd | Hospitality | 2025 |
| Olgoods Ltd | Wholesale & retail | 2025 |
| OLI Goss Ltd | Professional & scientific | 2021 |
| Oliabi Consultancy Limited | Information & communication | 2016 |
| Olim Cares Ltd | Health & social care | 2022 |
| Olive Air Ltd | Manufacturing | 2021 |
| Olive Kitchens Suffolk Ltd | Construction | 2023 |
| Olive Monkey Limited | Hospitality | 2020 |
| OLIVE247 Healthcare Limited | Health & social care | 2020 |
| Oliver Automotive Ltd | Wholesale & retail | 2022 |
| Oliver Legacy Limited | Real estate | 2021 |
| Oliver Marks Consulting Ltd. | Administrative & support | 2016 |
| Olivian Ostafe Ltd | Transport & logistics | 2023 |
| Olivine Capital Partners Limited | Professional & scientific | 2000 |
| Olivine Partners LLP | | 2004 |
| Ollie Turner Nutrition Ltd. | Arts & recreation | 2021 |
| Olympus Autocare Ltd | Wholesale & retail | 2016 |
| Olympus Mechanical Services Ltd | Construction | 2019 |
| OM27 Ltd | Arts & recreation | 2017 |
| Omag Logistics Limited | Transport & logistics | 2024 |
| Omar Bogdan Services Ltd | Wholesale & retail | 2024 |
| Omega Health Injury & Wellness Centre CIC | Health & social care | 2026 |
| OMEGA24 Ltd | Transport & logistics | 2026 |
| Omer BBQ House Ltd | Hospitality | 2018 |
| OMG HC Ltd | Wholesale & retail | 2026 |
| Ommo London Ltd | Manufacturing | 2017 |
| Omni Freight Services Ltd | Transport & logistics | 2010 |
| Omni Link Tech Ltd | Information & communication | 2026 |
| Omnicorp Limited | Real estate | 1993 |
| Omnikit Limited | Wholesale & retail | 2026 |
| Omnisco Ltd. | Real estate | 2001 |
| ON Fire EN Ltd | Hospitality | 2015 |
| ON Point Maintenance Ltd | Construction | 2020 |
| ONB Investments Ltd. | Real estate | 2025 |
| Oncobotanica Ltd | Wholesale & retail | 2019 |
| One Day Event Hire Limited | Administrative & support | 2024 |
| One Express Solutions Limited | Transport & logistics | 2022 |
| One Group Construction Limited | Professional & scientific | 2013 |
| One Planet Aesthetics Ltd | Professional & scientific | 2024 |
| One Planet Partnership Ltd | Wholesale & retail | 2022 |
| One Small Step Ltd | Wholesale & retail | 2011 |
| One Step Stronger Ltd | Information & communication | 2026 |
| One Stop Employment Ltd | Hospitality | 2020 |
| One Trick Ltd | Hospitality | 2025 |
| One Unique Services Ltd | Wholesale & retail | 2025 |
| Onik Trans Ltd | Transport & logistics | 2015 |
| Only Social Care Recruitment Ltd | Administrative & support | 2006 |
| Onspot Care Ltd | Administrative & support | 2023 |
| Onventure Ltd | Administrative & support | 2022 |
| Onyx Construction Consultants Limited | Professional & scientific | 2003 |
| Onyx Consultants Limited | Professional & scientific | 2009 |
| Onyx Holdings (East) Ltd | Financial services | 2021 |
| Onyx Solutions (East) Ltd | Construction | 2012 |
| OOH! Sore Feet? (EA) Ltd | Health & social care | 2015 |
| OOH! Sore Feet? Ltd | Health & social care | 2011 |
| Opacus Ltd | Information & communication | 2008 |
| Opal Bathrooms by Design Ltd. | Construction | 2008 |
| Opal Personnel Ltd | Administrative & support | 2023 |
| Open Access Limited | Information & communication | 1997 |
| Open Communications International Limited | Other services | 1983 |
| Opened Door Limited | Professional & scientific | 2013 |
| Openkontrol Ltd | Professional & scientific | 2022 |
| Openshelf UK Ltd | Transport & logistics | 2025 |
| Openvibe Limited | Information & communication | 2006 |
| Opervanta Usage Paths Ltd | Information & communication | 2026 |
| Opervo Ltd | Wholesale & retail | 2026 |
| OPO Media Limited | Professional & scientific | 2018 |
| Opportunities Included CIC | Education | 2014 |
| OPT Holdings Limited | Financial services | 2024 |
| OPT Services Limited | Professional & scientific | 1996 |
| Optal Solutions Limited | Information & communication | 2024 |
| Opti Route Global Limited | Transport & logistics | 2025 |
| Optics World Ltd | Wholesale & retail | 2024 |
| Optim Consultancy Limited | Professional & scientific | 2006 |
| Optima Carlton Colville Ltd | Extraterritorial | 2018 |
| Optima Health Solutions Limited | Health & social care | 2015 |
| Optima Land & Property Limited | Real estate | 2008 |
| Optima York Limited | Health & social care | 2009 |
| Optimal EAR Care Services Ltd | Health & social care | 2021 |
| Optimal Managed Solutions Limited | Professional & scientific | 2012 |
| Optimal Security Services Ltd | Financial services | 2025 |
| Optimistic Oasis Limited | Education | 2025 |
| Optimum Controls Services Limited | Construction | 1992 |
| Optimum Highway Services Ltd. | Construction | 2024 |
| OPU Ahmed Ltd | Wholesale & retail | 2025 |
| Opulent Crest Realty Ltd | Real estate | 2024 |
| Opus People Solutions (East) Limited | Administrative & support | 2016 |
| Opus People Solutions Group Limited | Administrative & support | 2014 |
| Opus People Solutions Limited | Administrative & support | 2017 |
| Opus Teach Limited | Administrative & support | 2017 |
| Oracle FX Ltd | Arts & recreation | 2023 |
| Orbital VIP Limited | Administrative & support | 2024 |
| Orbiter Systems Limited | Information & communication | 1996 |
| Orchard ROW Property Limited | Real estate | 2021 |