| Omni Therapy Limited | Information & communication | 2019 |
| Omnivue Media Group Limited | Information & communication | 2025 |
| OMO B Anointed Global Ltd | Health & social care | 2014 |
| OMX Media Limited | Other services | 2023 |
| ON (Essex) Limited | Wholesale & retail | 2012 |
| ON Point Build Ltd | Construction | 2025 |
| ON Site Electric Limited | Construction | 2015 |
| ON Site Plumbing & Electric Limited | Construction | 2015 |
| ON Virtual World Ltd | Information & communication | 2021 |
| Onacacia Ltd | Wholesale & retail | 2021 |
| Onari Integrated Care Ltd | Health & social care | 2024 |
| One Address Ltd | Administrative & support | 2024 |
| One Aesthetics Services Ltd | Health & social care | 2023 |
| One Build London Ltd | Administrative & support | 2019 |
| One Dental Kensington Limited | Health & social care | 2025 |
| One Dental Services Ltd | Health & social care | 2022 |
| One Dial Solutions Ltd | Administrative & support | 2026 |
| One EAR Arts Ltd | Information & communication | 2022 |
| One Executive Limited | Transport & logistics | 2013 |
| One Fifty Plus Ltd | Real estate | 2025 |
| One Frame Construction Limited | Construction | 2021 |
| One Interior Limited | Construction | 2016 |
| One Last One Ltd | Hospitality | 2025 |
| One Love Basildon Takeaway Limited | Hospitality | 2025 |
| One Pure Group Limited | Construction | 2018 |
| One Space Property Management Limited | Real estate | 2021 |
| One Step Property Group Limited | Real estate | 2021 |
| One Stop Furniture Store Ltd | Wholesale & retail | 2023 |
| One Stop Shop 7 Days Ltd | Wholesale & retail | 2025 |
| One Touch Youth Football Club Limited | Education | 2026 |
| One Way Insulation Ltd | Construction | 2018 |
| One Wellness Group Limited | Other services | 2024 |
| One World Nannies Ltd | Health & social care | 2009 |
| Oneness Solutions Consul Ltd | Health & social care | 2016 |
| Onerise Global Ltd | Administrative & support | 2026 |
| Onestopshopuk Ltd | Administrative & support | 2025 |
| Onform (Dies) Limited | Manufacturing | 1985 |
| Online Mastery Limited | Information & communication | 2012 |
| Only FAN Standing Limited | Arts & recreation | 2017 |
| Only Torques and Horses Limited | Wholesale & retail | 2022 |
| Onos Construction Ltd | Construction | 2023 |
| Onoscpower Limited | Transport & logistics | 2018 |
| Onsight Cleaning & Maintenance Ltd | Administrative & support | 2018 |
| Ontario Furniture Stores Limited | Wholesale & retail | 2017 |
| Onward IT Limited | Information & communication | 1999 |
| Onward Legal Services Ltd | Professional & scientific | 2026 |
| Onyx Medical Ltd | Health & social care | 2016 |
| Onyxoaks Limited | Real estate | 2022 |
| Ooosch Ltd | Other services | 2019 |
| OP Wheels Ltd | Wholesale & retail | 2024 |
| Opal BPM Limited | Administrative & support | 2013 |
| Openfields Counselling Clinic Ltd | Health & social care | 2013 |
| Openspaces Compliance Consultants Limited | Professional & scientific | 2010 |
| Openspaceuk Roofing Contractors Limited | Construction | 2020 |
| Opiair Conditioning Ltd | Construction | 2022 |
| Opid Studio Ltd | Professional & scientific | 2024 |
| Opies Holdings Limited | Other services | 2022 |
| Opies Stoves Limited | Wholesale & retail | 2018 |
| Oprah Solanke Limited | Health & social care | 2015 |
| Oprha Dogs Ltd | Hospitality | 2018 |
| Opti Life Wellbeing Ltd | Wholesale & retail | 2026 |
| Optimal Solutions Consultancy Ltd | Administrative & support | 2023 |
| Optimum Moving Solutions Limited | Transport & logistics | 2022 |
| Optimus UK Logistics Ltd | Transport & logistics | 2019 |
| Options Construction Services Ltd | Construction | 2020 |
| Options Event Management Ltd | Hospitality | 2022 |
| Optionsx Ltd | Information & communication | 2024 |
| OR Consultancy Ltd | Professional & scientific | 2024 |
| Orange Abode Limited | Real estate | 2021 |
| Orbis Exchange Group Limited | Financial services | 2018 |
| Orbis Exchange Properties Limited | Real estate | 2020 |
| Orbis Partner Group Limited | Information & communication | 2018 |
| Orbit MSP Ltd | Information & communication | 2024 |
| Orbit Nexus Limited | Administrative & support | 2024 |
| Orbita Funding 2022-1 PLC | Financial services | 2021 |
| Orbitplay Technologies Ltd | Information & communication | 2026 |
| Orbuilding Limited | Construction | 2025 |
| Orca Dive Store Limited | Wholesale & retail | 2019 |
| Orca Scuba Diving Academy Limited | Wholesale & retail | 2017 |
| Orchard Estates (Essex) Limited | Construction | 1957 |
| Orchard-Rowe Consultancy Limited | Administrative & support | 2014 |
| Orchard-Rowe Property Ltd | Real estate | 2020 |
| Orchards Holdings Ltd | Financial services | 2021 |
| Organipom Ltd | Wholesale & retail | 2026 |
| Orion Support Ltd | Health & social care | 2022 |
| Orions Logistics Ltd | Transport & logistics | 2026 |
| Orison Tours Ltd | Administrative & support | 2025 |
| Oriss Ltd | Transport & logistics | 2017 |
| Ormesby Estates Limited | Extraterritorial | 2022 |
| Ornel Health Limited | Health & social care | 2020 |
| Orwell Properties Limited | Real estate | 2023 |
| Osarch Environmental Ltd | Water & waste | 2023 |
| OSG Europe Limited | Professional & scientific | 1995 |
| OSG U.K. Limited | Manufacturing | 1969 |
| Oshoja Commerce Ltd | Wholesale & retail | 2025 |
| Oshoja Properties Ltd | Real estate | 2025 |
| Osney Estates (Holdings) Limited | Professional & scientific | 2012 |
| Osney Estates Limited | Real estate | 1972 |
| OSS Plumbing & Heating Ltd | Construction | 2026 |
| OTB Margate Ltd | Other services | 2020 |