| One Two Key Properties Limited | Real estate | 2019 |
| One-Eighty Limited | Education | 2011 |
| Onemind Awareness Ltd. | Other services | 1997 |
| Ongolo Limited | Information & communication | 2021 |
| Onik DOM Ltd | Real estate | 2020 |
| Onist Food Limited | Hospitality | 2015 |
| Online Property UK Ltd | Real estate | 2022 |
| Only the Brave Films Limited | Administrative & support | 2014 |
| Onlyus Enterprises UK Ltd | Wholesale & retail | 2019 |
| Onroute Couriers & Logistics Ltd | Transport & logistics | 2026 |
| Onscene Medicals Services Limited | Health & social care | 2025 |
| Onsol Ltd | Professional & scientific | 2017 |
| Onyen Enterprises Ltd | Construction | 2025 |
| Ooak Property Limited | Real estate | 2021 |
| Ooex Limited | Professional & scientific | 2023 |
| OP Holdings Limited | Financial services | 2004 |
| Opac (Catering) Limited | Real estate | 1982 |
| Open Code Ltd | Information & communication | 2022 |
| Open Imagination Ltd | Information & communication | 2013 |
| Open Plan Law Limited | Professional & scientific | 2016 |
| Open Roads for Oxford Ltd | Other services | 2025 |
| Open Sailor Ltd | Education | 2023 |
| Open Sky Properties Limited | Real estate | 2022 |
| Openclinical C.I.C. | Professional & scientific | 2015 |
| Opendoorz Limited | Administrative & support | 2010 |
| Openwell Ltd | Professional & scientific | 2018 |
| Opera Catering Ltd | Hospitality | 2018 |
| Operation Fistula (UK) | Health & social care | 2015 |
| Opes Borsa Technologies Limited | Information & communication | 2025 |
| OPL Holdings Limited | Financial services | 2020 |
| OPM (Oxford) Limited | Construction | 2017 |
| OPP Programme Office Ltd | Education | 2014 |
| Oppday Ltd | Health & social care | 2021 |
| Oppenheimer's Ltd | Real estate | 2017 |
| Oppi Advisory Limited | Professional & scientific | 2025 |
| Opportunity International United Kingdom | Professional & scientific | 2005 |
| Opportunity Knocks Direct Ltd | Other services | 2001 |
| Opportunity Microfinance Investments Limited | Professional & scientific | 2003 |
| OPS Corp Ltd. | Extraterritorial | 2023 |
| Opsydia Limited | Professional & scientific | 2017 |
| Optek Limited | Manufacturing | 2000 |
| Optellum Ltd | Professional & scientific | 2015 |
| Optergy Ltd | Professional & scientific | 2019 |
| Optibiteai CIC | Information & communication | 2025 |
| Optical View Limited | Health & social care | 2011 |
| Optimapharm (UK) Limited | Professional & scientific | 2018 |
| Optimentis Consulting Limited | Professional & scientific | 2024 |
| Optimist Works Limited | Professional & scientific | 2020 |
| Opulence CPM Limited | Professional & scientific | 2019 |
| Oqema Limited | Manufacturing | 1975 |
| ORA Prep Limited | Education | 2013 |
| Oralift Limited | Professional & scientific | 2002 |
| Orange Blossom Properties Limited | Real estate | 2019 |
| Orangeglobal Business Services GmbH | | 2007 |
| Oranje & Oak Limited | Financial services | 2025 |
| Orbane Group Limited | Hospitality | 2025 |
| Orbis Nexus Advisory Ltd | Professional & scientific | 2026 |
| Orbit Discovery Group Ltd | Professional & scientific | 2020 |
| Orbit Discovery Ltd | Professional & scientific | 2020 |
| Orbit Energy Limited | Wholesale & retail | 2025 |
| Orbital Art Limited | Arts & recreation | 2022 |
| Orchard Design and Construction Limited | Extraterritorial | 2014 |
| Orchard Developments (Bucks) Limited | Construction | 2014 |
| Orchard Therapeutic Care Ltd | Health & social care | 2024 |
| Ordained Path Integrated (UK) Limited | Other services | 2025 |
| Orfc Trading Ltd | Hospitality | 2023 |
| Orfonyx BIO Ltd | Professional & scientific | 2022 |
| Organisations Work Ltd | Professional & scientific | 2015 |
| Organum Ltd. | Administrative & support | 1998 |
| Oribeck Ltd | Real estate | 2022 |
| Oriel College Conferences Limited | Hospitality | 2011 |
| Oriel Homes (Bisham) Limited | Construction | 2022 |
| Oriel Homes (Marston) Limited | Construction | 2019 |
| Oriel Land Limited | Real estate | 2017 |
| Oriel Square Limited | Information & communication | 2017 |
| Orient Tattoo Limited | Wholesale & retail | 2006 |
| Origin Transport Consultants Limited | Professional & scientific | 2013 |
| Originai Limited | Information & communication | 2023 |
| Original Field of Architecture Limited | Professional & scientific | 2008 |
| Orikami Limited | Professional & scientific | 2015 |
| Orion Belt Industries Limited | Information & communication | 2026 |
| Orion Vortex Limited | Wholesale & retail | 2025 |
| Oriono Digital Limited | Information & communication | 2023 |
| Orlock Bay Limited | Administrative & support | 2021 |
| Orovel Ltd | Professional & scientific | 2020 |
| Orpwood Limited | Real estate | 1997 |
| Orthanc Limited | Information & communication | 2025 |
| Ortho International Limited | Wholesale & retail | 2019 |
| Orthopaedic Specialist Services Ltd | Professional & scientific | 2024 |
| Orthoson Limited | Manufacturing | 2015 |
| Osaat Care Homes Ltd | Health & social care | 2025 |
| OSE Labs Limited | Financial services | 2024 |
| OSE Manager Limited | Financial services | 2024 |
| Osecare Limited | Health & social care | 2019 |
| Osena Limited | Wholesale & retail | 2012 |
| Osep CIC | Other services | 2019 |
| OSF Manco Limited | Professional & scientific | 2026 |
| OSI Midco Limited | Financial services | 2020 |
| Osler Diagnostics Limited | Professional & scientific | 2016 |
| Osler Employee Holdings Limited | Financial services | 2025 |