| Oneepsilon Ltd | Professional & scientific | 2024 |
| Oneformatics Ltd | Information & communication | 2016 |
| Onegift AI Ltd | Education | 2024 |
| Oneill Aero Ltd | Manufacturing | 2026 |
| Oneleap Limited | Professional & scientific | 2009 |
| Onesound Media Ltd | Information & communication | 2023 |
| Onespacemedia Ltd. | Information & communication | 2008 |
| Onimations Ltd | Information & communication | 2023 |
| Onley Technical Consulting Ltd | Information & communication | 2011 |
| Online Media Limited | Wholesale & retail | 2007 |
| Online Rehab Ltd | Other services | 2024 |
| Online Southampton Limited | Education | 2023 |
| Only KOI Ltd | Agriculture & fishing | 2026 |
| ONLY3PALS Limited | Wholesale & retail | 2025 |
| Onniel Ltd | Professional & scientific | 2023 |
| Onstream Communications Limited | Information & communication | 2006 |
| Onyva Agency Limited | Professional & scientific | 2014 |
| Onyx Technology Services Ltd | Information & communication | 2015 |
| Onzo Limited | Information & communication | 2007 |
| Opal Court Management Limited | Real estate | 2003 |
| Opal Street UK Limited | Wholesale & retail | 2026 |
| Opas Consultancy Limited | Information & communication | 2013 |
| Opas Properties Limited | Real estate | 2019 |
| Opaviation Ltd | Manufacturing | 2025 |
| Open Book Publishers Community Interest Company | Information & communication | 2008 |
| Open Clearing Limited | Information & communication | 2024 |
| Open Climb Ltd | Education | 2019 |
| Open Design Agency Limited | Professional & scientific | 2018 |
| Open Impact Ltd | Information & communication | 2019 |
| Open Mosaic Ltd | Information & communication | 2025 |
| Open Ocean Tackle Ltd | Wholesale & retail | 2014 |
| Open Source Management Ltd | Information & communication | 2012 |
| Open Systems Limited | Professional & scientific | 1984 |
| Open World Ltd | Extraterritorial | 2022 |
| Openarch Estates Limited | Real estate | 1999 |
| Openarch Greenside Limited | Real estate | 1996 |
| Openarch Holdings 2025 Ltd | Financial services | 2025 |
| Openarch Properties Limited | Real estate | 2013 |
| Openframeworks Limited | Professional & scientific | 2014 |
| Openinterx Ltd | Information & communication | 2024 |
| Openkit Ltd | Information & communication | 2020 |
| Openovo Publishing Limited | Information & communication | 2024 |
| Openstreetmap Foundation | Information & communication | 2006 |
| Opentrack Events Limited | Arts & recreation | 2010 |
| Opentrust | Education | 2000 |
| Operation Mobile Ltd | Information & communication | 2011 |
| Opeworld Technology Co., Ltd | Wholesale & retail | 2021 |
| Ophelia Import & Export Co., Ltd | Extraterritorial | 2012 |
| Ophrys Solutions Limited | Information & communication | 2006 |
| Opinio Group Limited | Administrative & support | 2011 |
| Oppo Electronics Ltd | Wholesale & retail | 2009 |
| Optalay Limited | Professional & scientific | 2017 |
| Optatum Ltd | Professional & scientific | 2019 |
| Optburn Ltd | Information & communication | 2025 |
| Optibrium Bidco Limited | Financial services | 2021 |
| Optibrium Limited | Information & communication | 2008 |
| Optibrium Midco Limited | Administrative & support | 2021 |
| Optibrium Topco Limited | Financial services | 2021 |
| Optim Forensic Engineers Limited | Professional & scientific | 2010 |
| Optimal Carers Limited | Health & social care | 2020 |
| Optimal Optom Ltd | Administrative & support | 2023 |
| Optimaze Ltd | Wholesale & retail | 2023 |
| Optimum Detail Ltd | Wholesale & retail | 2017 |
| Optimum Patient Care Limited | Health & social care | 2016 |
| Optimum Personal Training Limited | Arts & recreation | 2017 |
| Optimum REG Associates Ltd | Professional & scientific | 2014 |
| Optimus Commercials and Engineering Limited | Other services | 2021 |
| Optio (UK) Limited | Administrative & support | 2013 |
| Optisynx Limited | Manufacturing | 2006 |
| Optivium Ltd | Information & communication | 2025 |
| Optiwell Medical Limited | Education | 2026 |
| Optocopia Ltd | Professional & scientific | 2021 |
| Opuluxe&Co Ltd | Wholesale & retail | 2020 |
| Opus Flux Limited | Information & communication | 2023 |
| Opus Instruments Ltd | Manufacturing | 2007 |
| Oqtave FLY Innovation Ltd | Information & communication | 2023 |
| OR Consult Ltd | Information & communication | 2025 |
| Oracle Gold Resources Ltd | Mining & quarrying | 2020 |
| Oracle Power PLC | Mining & quarrying | 2006 |
| Oraclum Intelligence Systems Ltd | Information & communication | 2016 |
| Orange DEN Ltd | Professional & scientific | 2015 |
| Orange DOT Properties Limited | Real estate | 2017 |
| Orange Goose Ltd. | Construction | 2017 |
| Orangetree Property Management Limited | Households | 1982 |
| Oranier Ltd | Extraterritorial | 2021 |
| Orbit Medical Communications Ltd | Professional & scientific | 2014 |
| Orbit Talent Management Ltd | Professional & scientific | 2025 |
| Orbita Consultants Ltd | Professional & scientific | 2019 |
| Orbol Ltd | Real estate | 2023 |
| Orca Scan Limited | Information & communication | 2016 |
| Orca Security UK Ltd | Information & communication | 2021 |
| Orca Telemetry Ltd | Information & communication | 2026 |
| Orchard Accounting Solutions Limited | Professional & scientific | 2009 |
| Orchard Green Estate Management Company Limited | Households | 2019 |
| Orchard Park Vapes Limited | Wholesale & retail | 2022 |
| Orchard Property Group Ltd | Wholesale & retail | 2017 |
| Orchard Publishing Co Limited | Administrative & support | 1992 |
| Orchard Road Barkway Limited | Households | 2017 |
| Orchard Studio Ltd | Professional & scientific | 2007 |
| Orchard Tea Garden Limited | Hospitality | 2010 |