| Onsite Education Services Ltd | Education | 2023 |
| Ontrack Computers Limited | Information & communication | 2007 |
| Onyx Freehold Investments Limited | Real estate | 2024 |
| Onyx One Intelligence Ltd | Professional & scientific | 2025 |
| Onyx Property Group CIC | Real estate | 2025 |
| Onyx Resin Group Ltd | Construction | 2025 |
| Onyx Site OPS Ltd | Construction | 2019 |
| Onyx Social Housing Ltd | Real estate | 2026 |
| OOE Ltd | Real estate | 2019 |
| Oogi Investments Limited | Real estate | 1964 |
| OPACITY365 Limited | Construction | 2019 |
| Opal Contracts (U.K.) Limited | Construction | 2005 |
| Opendoor Housing Limited | Real estate | 2025 |
| Openstreetmap United Kingdom Community Interest Company | Information & communication | 2016 |
| Opex Resolutions Ltd | Professional & scientific | 2023 |
| Ophthalmology Innovations Ltd. | Health & social care | 2012 |
| Opinion Expert | Information & communication | 2019 |
| Oppidan Inns Ltd | Hospitality | 2015 |
| Oprayo Ltd | Information & communication | 2025 |
| Optimal Fitness Ltd | Arts & recreation | 2023 |
| Optimal Health UK - Wilday Consultancy Ltd | Health & social care | 2020 |
| Optimal Worldwide Ltd | Arts & recreation | 2024 |
| Optimark Ltd | Administrative & support | 2012 |
| Optimum Automation Limited | Professional & scientific | 2018 |
| Optimum Integration Ltd | Information & communication | 2020 |
| Optimum Talent Management Limited | Administrative & support | 2025 |
| Opulent Studios Limited | Information & communication | 2025 |
| Opus Production House Ltd. | Information & communication | 2022 |
| Opus Promotions & Marketing Limited | Wholesale & retail | 2010 |
| OPX Engineering Ltd | Manufacturing | 2024 |
| Orange 7 AI Limited | Information & communication | 2024 |
| Orange and Gray Limited | Information & communication | 2017 |
| Orangestar Bristol Street Commercial Limited | Real estate | 2025 |
| Orangestar Capital (Bedford Arcade) Limited | Real estate | 2012 |
| Orangestar Capital (Bristol Street ) Holdings Limited | Real estate | 2019 |
| Orangestar Capital (Droitwich) Limited | Real estate | 2023 |
| Orangestar Capital (Globe Bristol) Limited | Real estate | 2012 |
| Orangestar Capital (Jersey) Limited | Real estate | 2012 |
| Orangestar Capital (Lancaster Street) Limited | Real estate | 2019 |
| Orangestar Capital (Poole) Limited | Real estate | 2013 |
| Orangestar Capital (Portsmouth) Limited | Real estate | 2010 |
| Orangestar Capital (Slindon Portsmouth) Limited | Real estate | 2011 |
| Orangestar Capital (Solihull) Ltd | Real estate | 2018 |
| Orangestar Capital (ST Philips) Limited | Real estate | 2019 |
| Orangestar Capital (Whittaker) Limited | Real estate | 2010 |
| Orangestar Capital Portsmouth Car Parks Limited | Real estate | 2025 |
| Orangestar Capital Rugby Limited | Real estate | 2019 |
| Oraserv Ltd. | Information & communication | 2009 |
| Oray Investments Limited | Administrative & support | 2021 |
| Orbit Health Services Ltd | Health & social care | 2017 |
| Orbs Energy Ltd | Construction | 2011 |
| Orca Software Solutions Limited | Information & communication | 2013 |
| Orchard Capital Limited | Professional & scientific | 2017 |
| Orchard Care Limited | Construction | 2019 |
| Orchard Court Management (Leamington Spa) Limited | Households | 1970 |
| Orchard View Construction Limited | Construction | 2024 |
| Orchard View Holdings Ltd | Households | 2024 |
| Orchid Computers Limited | Information & communication | 2008 |
| Orci Trials Limited | Professional & scientific | 2018 |
| Ordinary Magic C.I.C. | Education | 2019 |
| Ordo Consultancy Limited | Information & communication | 2011 |
| Organic Learners Ltd | Education | 2021 |
| Orgbar Aluminium Limited | Manufacturing | 2008 |
| Oriana Store Ltd | Wholesale & retail | 2026 |
| ORIGIN8Z Limited | Administrative & support | 2021 |
| Orish Ltd | Wholesale & retail | 2020 |
| Orland Park LP | | 2014 |
| Orthodontic Excellence Limited | Health & social care | 2019 |
| Orthodontics for YOU (Birmingham) Limited | Health & social care | 2019 |
| Orthodontics for YOU (Bromsgrove) Limited | Health & social care | 2015 |
| Orthodontics for YOU (Solihull) Limited | Health & social care | 2015 |
| Orthodontics for YOU (West Midlands) Limited | Health & social care | 2019 |
| Orthodontics for YOU (Worcestershire) Limited | Health & social care | 2019 |
| Orthodontics for YOU Limited | Health & social care | 2011 |
| Orthomasters Limited | Health & social care | 2010 |
| Orthoscan Limited | Health & social care | 2020 |
| Ortus Estates Limited | Real estate | 2010 |
| OS Management Limited | Administrative & support | 2013 |
| Osborne Commercial Properties Limited | Manufacturing | 2006 |
| Osborne Fasteners Limited | Manufacturing | 2006 |
| Osborne Fasteners Manufacturing Ltd | Manufacturing | 2018 |
| Osbornsaviours Ltd | Wholesale & retail | 2026 |
| Oscar Consultants Limited | Real estate | 1995 |
| OSJ Investments Limited | Financial services | 2021 |
| Oslo Investments Limited | Real estate | 1998 |
| Osloai Ltd | Information & communication | 2026 |
| Osman Consulting Ltd | Professional & scientific | 2013 |
| OSO Juicee Limited | Manufacturing | 2026 |
| OSP Investments Limited | Real estate | 2019 |
| Ossr Properties Ltd | Real estate | 2020 |
| Ostra Consulting Limited | Information & communication | 2021 |
| OT15 Ltd | Construction | 2022 |
| OTA Transport Ltd | Transport & logistics | 2022 |
| OTC Finance Limited | Financial services | 2019 |
| Otob Limited | Professional & scientific | 2018 |
| OTS Consulting Limited | Professional & scientific | 2024 |
| Otta Cleaning Solutions Ltd | Administrative & support | 2025 |
| Oughton Holdings Ltd | Financial services | 2021 |
| OUR Core Ltd | Health & social care | 2022 |
| OUR Headspace Limited | Health & social care | 2015 |