| Omicron (UK) Limited | Other services | 1988 |
| Omixion Tech Solutions Ltd | Information & communication | 2025 |
| OMM Group Holdings Ltd | Financial services | 2022 |
| Omni Intelligence Limited | Information & communication | 1999 |
| Omnicosmos Ltd | Financial services | 2023 |
| Omnisoft Technologies Ltd | Information & communication | 2004 |
| Omsai Consulting Ltd | Information & communication | 2015 |
| ON Point Coding Ltd | Wholesale & retail | 2021 |
| ON Track PPF Ltd | Transport & logistics | 2024 |
| On-Time Building Supply Limited | Construction | 2013 |
| Ondagrass Football Scouting Network Limited | Education | 2025 |
| One 2 One Hire Solutions Ltd | Transport & logistics | 2024 |
| One Camelot Biocapital Limited | Manufacturing | 2025 |
| One Click Cover Limited | Financial services | 2017 |
| One Forterra Srpo Real Estate Ltd | Financial services | 2026 |
| One Garden Limited | Wholesale & retail | 2014 |
| One Global Spirits Ltd | Manufacturing | 2025 |
| One Keble Road Management Limited | Households | 2000 |
| One Nation Consulting Ltd | Professional & scientific | 2013 |
| One Synergy Partners IHC LLP | | 2026 |
| One Touch Data Limited | Information & communication | 2008 |
| One Twenty Ltd | Professional & scientific | 2006 |
| Onetruthmodel Ltd | Information & communication | 2019 |
| Onguls Limited | Wholesale & retail | 2025 |
| Online Super Bargains Ltd | Wholesale & retail | 2022 |
| Onoroff Limited | Information & communication | 2013 |
| ONR Training Consultants Ltd | Information & communication | 2008 |
| Onthree Limited | Wholesale & retail | 2025 |
| Onymax Ltd | Administrative & support | 2023 |
| Onyx Group Limited | Extraterritorial | 2006 |
| Onyx Information Technology Holdings Limited | Extraterritorial | 2011 |
| Onyx Internet Limited | Extraterritorial | 2000 |
| Onyx Trustees Limited | Administrative & support | 2015 |
| Onyx UK Group Limited | Construction | 2021 |
| Oomph Works Ltd | Information & communication | 2022 |
| Open and Shut Doors Ltd | Construction | 2026 |
| Open Flowmesh Consultants Ltd | Information & communication | 2026 |
| Openheath Limited | Professional & scientific | 1998 |
| Opexx Limited | Information & communication | 2023 |
| Oppenheim Limited | Professional & scientific | 2023 |
| Opperman Investments Limited | Real estate | 1940 |
| Optex (Europe) Limited | Wholesale & retail | 1991 |
| Optima Podiatry Ltd | Health & social care | 2024 |
| Optimal Solutions Limited | Construction | 2025 |
| Option 5 Health Limited | Administrative & support | 2022 |
| Optipro Solutions Limited | Administrative & support | 2019 |
| Opulence Cleaning Solutions Ltd | Administrative & support | 2025 |
| Opusludus Ltd | Information & communication | 2017 |
| Orange House Media Ltd | Information & communication | 2025 |
| Orbis Syndicate Limited | Financial services | 2025 |
| Orbit Familia Studios Ltd | Information & communication | 2025 |
| Orca Service Technologies IP Limited | Financial services | 2021 |
| Orca Service Technologies Limited | Information & communication | 2021 |
| Orchard END Advisory Limited | Professional & scientific | 2021 |
| Orchard View Projects Limited | Real estate | 2025 |
| Orchid Residency Ltd | Real estate | 2019 |
| Order Shoppe Ltd | Wholesale & retail | 2023 |
| Orenki Ltd | Professional & scientific | 2019 |
| Organised Platform Ltd | Information & communication | 2025 |
| Original Productions Limited | Arts & recreation | 1989 |
| Orion Property Group Ltd | Construction | 2003 |
| Orion Scenery Limited | Construction | 2014 |
| Orison Consultancy Limited | Professional & scientific | 2017 |
| Ormond Developments Limited | Real estate | 2010 |
| Orthofix Limited | Wholesale & retail | 1993 |
| Orzax Limited | Wholesale & retail | 2021 |
| Osbourne Purdie Limited | Arts & recreation | 1995 |
| Oscar Flohr Films Ltd | Information & communication | 2025 |
| Ospray Airless Limited | Construction | 2014 |
| Osscli Ltd | Professional & scientific | 2025 |
| Ossvisor Limited | Information & communication | 2010 |
| Osztech Ltd | Wholesale & retail | 2019 |
| OUR Baby Club Limited | Health & social care | 2018 |
| Outbuild Homes Ltd | Construction | 2023 |
| Outstanding Early Years Limited | Administrative & support | 2025 |
| Outstepperz Global Ltd | Administrative & support | 2024 |
| Oval (2284) Limited | Financial services | 2014 |
| Ovenique Ltd | Hospitality | 2026 |
| Overseas Direct Investment Limited | Wholesale & retail | 1992 |
| Ovolv Limited | Professional & scientific | 2025 |
| Owen Training Services Limited | Administrative & support | 2020 |
| Owenstone Property Limited | Real estate | 2025 |
| OX Tech Consultancy Limited | Information & communication | 2016 |
| Oxford and Cambridge I.A. Limited | Financial services | 2002 |
| Oxford Business Psychology Ltd. | Professional & scientific | 2016 |
| Oxford Research Co Ltd | Wholesale & retail | 2015 |
| Oxford Resource Management Limited | Financial services | 2010 |
| Oxipro Medical Ltd | Manufacturing | 2022 |
| Oxstem Cardio Limited | Professional & scientific | 2016 |
| Oxstem Limited | Professional & scientific | 2011 |
| Oxstem Neuro Limited | Professional & scientific | 2016 |
| Oxstem Oncology Limited | Professional & scientific | 2016 |
| Ozone Digital Ltd | Information & communication | 2026 |
| P & J Langton Property LLP | | 2015 |
| P Alderman Ltd. | Professional & scientific | 2015 |
| P K Construction London Ltd | Construction | 2018 |
| P Khan Enterprise Limited | Real estate | 2023 |
| P&A Building Ltd | Construction | 2015 |
| P&G Accounts and Tax Ltd | Professional & scientific | 2018 |
| P.A.W Supplies and Installations Ltd | Manufacturing | 2022 |