| Oneil Coaching Extra Ltd | Administrative & support | 2024 |
| Onesrv Ltd | Information & communication | 2026 |
| ONETOUCH247MAXVUE Limited | Information & communication | 2021 |
| ONETOUCH247TAPVUE Limited | Information & communication | 2021 |
| Onetouchmax Limited | Information & communication | 2021 |
| Onetouchmaxvue Limited | Information & communication | 2021 |
| Onetouchmoment Limited | Information & communication | 2021 |
| Onetouchtap Limited | Information & communication | 2021 |
| Onetouchtapvue Limited | Information & communication | 2021 |
| Onetouchtruetech Limited | Information & communication | 2021 |
| Onetwentytwo Consulting Limited | Information & communication | 2014 |
| Oneuponedown Limited | Information & communication | 2023 |
| Onevision Contractors Ltd | Construction | 2024 |
| Onevision Software (UK) Limited | Information & communication | 1999 |
| Onezulusevenzero Limited | Extraterritorial | 2013 |
| Onkar One Limited | Other services | 2023 |
| Online Coaching Ltd | Administrative & support | 2021 |
| Online Simplicity Ltd. | Wholesale & retail | 2018 |
| Only Gold Limited | Wholesale & retail | 2005 |
| Onna's Beauty Destination Ltd | Other services | 2022 |
| Onofrei Transport Ltd | Transport & logistics | 2022 |
| Onpoint Support Services Limited | Administrative & support | 2025 |
| Onsite Vending Ltd | Hospitality | 2026 |
| Ontime Business Turnaround Services Limited | Professional & scientific | 2020 |
| Ontime Logistics Europe Limited | Transport & logistics | 2009 |
| Ontime Logistics Group Ltd | Transport & logistics | 2000 |
| Ontime Response Healthcare Limited | Health & social care | 2018 |
| Ontisk Group Limited | Wholesale & retail | 2025 |
| ONU Trans Limited | Transport & logistics | 2020 |
| ONV Ltd | Construction | 2023 |
| Onwards Expeditions Limited | Arts & recreation | 2025 |
| Onyx Financial Investments Ltd | Real estate | 2026 |
| Onyx Financial Ltd | Financial services | 2024 |
| Onyx Financial Solutions Ltd | Financial services | 2026 |
| Onyx Financial Wills Ltd | Financial services | 2021 |
| Onyx Recruitment Associates Limited | Administrative & support | 2020 |
| Onyxcedar Limited | Health & social care | 2014 |
| Oo&A Ltd | Information & communication | 2024 |
| Ooclean & Co Limited | Administrative & support | 2024 |
| OOH LA LA Truly Events Ltd | Hospitality | 2025 |
| Oooh! Ltd | Manufacturing | 2026 |
| OOS89 Marine Consultancy Limited | Professional & scientific | 2024 |
| Oosphere Limited | Administrative & support | 2025 |
| OP Soft Limited | Information & communication | 2005 |
| Opagus Marketing Limited | Professional & scientific | 2003 |
| Opal Computers Limited | Transport & logistics | 1997 |
| Opal Home Care Ltd | Education | 2022 |
| Opal Ventures Limited | Professional & scientific | 2010 |
| Opallus Limited | Information & communication | 2014 |
| Opaléya Ltd | Manufacturing | 2025 |
| Opc-Pr Limited | Other services | 2022 |
| Open Datacentre Services Limited | Real estate | 2019 |
| Open Digi Solutions Limited | Information & communication | 2020 |
| Open Doors Property Group Ltd | Real estate | 2022 |
| Open Innovations Group Limited | Administrative & support | 2022 |
| Open Sky Coaching Ltd | Administrative & support | 2020 |
| Open University Business School Limited | Extraterritorial | 1994 |
| Open University Educational Enterprises Limited | Education | 1996 |
| Open University Enterprises Limited | Extraterritorial | 2002 |
| Open University International Ltd | Extraterritorial | 1996 |
| Open University Student Budget Accounts Limited | Financial services | 1995 |
| Open University Worldwide Learning Ltd | Extraterritorial | 1996 |
| Open-Projects Limited | Information & communication | 1989 |
| Openg Ltd | Professional & scientific | 2021 |
| Opening the Archives Ltd | Arts & recreation | 2022 |
| Openrich UK Limited | Health & social care | 2010 |
| Openroute Express Ltd | Transport & logistics | 2025 |
| Openwebcreations Limited | Information & communication | 2013 |
| Opera Solar Data Intelligence Ltd | Information & communication | 2022 |
| Ophthalmic Care Ltd | Health & social care | 2020 |
| Opportunity Knox Limited | Professional & scientific | 2001 |
| OPS Comms Solutions Limited | Information & communication | 2017 |
| Opstax Ltd | Information & communication | 2015 |
| Optic Art Studio Ltd | Professional & scientific | 2025 |
| Optieats Limited | Information & communication | 2025 |
| Optiestates Limited | Real estate | 2025 |
| Optiflow Logistics Ltd | Transport & logistics | 2023 |
| Optiim Ltd | Information & communication | 2022 |
| Optikore Limited | Wholesale & retail | 2025 |
| Optiletz Limited | Real estate | 2023 |
| Optim Consulting Services Ltd | Professional & scientific | 2020 |
| Optimal Delivery Limited | Professional & scientific | 2017 |
| Optimal Gynaecological Services Ltd | Health & social care | 2019 |
| Optimal R.A.D.S Ltd | Professional & scientific | 2021 |
| Optimind Technologies Ltd | Information & communication | 2025 |
| Optimum External Cleaning Services Limited | Administrative & support | 2021 |
| Optimum Haven Care Ltd | Health & social care | 2021 |
| Optimum-Linkz Engineering Limited | Professional & scientific | 2024 |
| Optimystic Care Ltd | Public administration | 2025 |
| Optinetz Limited | Information & communication | 2012 |
| Optino Lighting Ltd | Manufacturing | 2021 |
| Options Corporate Pensions UK Limited | Professional & scientific | 2014 |
| Options UK Personal Pensions LLP | | 2009 |
| Optitree Ltd | Information & communication | 2021 |
| Optyrc Solutions Ltd | Manufacturing | 2019 |
| Opulent Apparel Ltd | Wholesale & retail | 2024 |
| Opus Advisory Group Limited | Other services | 2025 |
| Opus Business Advisory Group Limited | Other services | 2017 |
| Opus Business Projects Limited | Other services | 2015 |
| Opus Business Services Limited | Professional & scientific | 2026 |