| Oakleigh Court Management (Whetstone) Limited | Households | 1983 |
| Oakmere Management Services Ltd | Real estate | 2021 |
| Oaktrades Ltd | Education | 2009 |
| Oakwood Building Services (London) Limited | Construction | 2022 |
| Oakwood Plant Sales and Hire Ltd | Manufacturing | 2021 |
| Oasis Land Management Ltd | Professional & scientific | 2025 |
| Ocado Central Services Limited | Professional & scientific | 2014 |
| Ocado Group PLC | Wholesale & retail | 2009 |
| Ocado Holdings Limited | Professional & scientific | 2010 |
| Ocado Innovation Limited | Information & communication | 2013 |
| Ocado Intelligent Automation Limited | Information & communication | 2023 |
| Ocado Operating Limited | Real estate | 2014 |
| Ocado Retail Limited | Wholesale & retail | 1999 |
| Ocado Solutions Limited | Information & communication | 2001 |
| Ocado Ventures (80 Acres) Limited | Professional & scientific | 2019 |
| Ocado Ventures (Inkbit) Limited | Professional & scientific | 2019 |
| Ocado Ventures (JFC) Limited | Professional & scientific | 2019 |
| Ocado Ventures (Karakuri) Limited | Professional & scientific | 2018 |
| Ocado Ventures (Myrmex) Limited | Professional & scientific | 2020 |
| Ocado Ventures (Oxbotica) Limited | Professional & scientific | 2020 |
| Ocado Ventures (Wayve) Limited | Professional & scientific | 2021 |
| Ocado Ventures Holdings Limited | Professional & scientific | 2015 |
| Oceanic Herbs Ltd | Wholesale & retail | 2023 |
| Octavian Trans Ltd | Transport & logistics | 2019 |
| Oddd Studio Limited | Arts & recreation | 2017 |
| Odefex Limited | Real estate | 2024 |
| Odus & Odus Limited | Households | 2025 |
| Odusote Properties UK Limited | Hospitality | 2015 |
| Odyssey Glory Mill Limited | Professional & scientific | 1999 |
| Odyssey Grips Limited | Information & communication | 2022 |
| Ofoq Trading Limited | Wholesale & retail | 2025 |
| Ogeds Foundation | Other services | 2020 |
| OHM Bookkeeping Services Limited | Professional & scientific | 2010 |
| Okopn Ltd | Extraterritorial | 2021 |
| Okout Ltd | Information & communication | 2020 |
| Oktoberfest Tour Limited | Hospitality | 2026 |
| Olam Properties Limited | Real estate | 2011 |
| Olatony Services Limited | Health & social care | 2008 |
| Olayfis Global Resources Ltd | Wholesale & retail | 2021 |
| Old Church Residential Management Limited | Households | 2010 |
| Old Mill Gardens Management Company Limited | Other services | 1988 |
| Old School Walk Limited | Households | 2004 |
| Olima Communications Limited | Administrative & support | 2015 |
| Oliver Building Services Ltd | Construction | 2021 |
| Olivia Pro Transport Ltd | Transport & logistics | 2025 |
| Olympus Mobile Tyre Fitting Hatfield Ltd | Wholesale & retail | 2023 |
| Omicron Projects Limited | Health & social care | 2004 |
| One Click Cars Ltd | Wholesale & retail | 2020 |
| One Life Wealth Planning Ltd | Financial services | 2010 |
| One Take Design Ltd | Information & communication | 2019 |
| One World Book Co. Ltd. | Extraterritorial | 1996 |
| Onecall Plumbing Ltd | Construction | 2024 |
| Ooni of IFE Global Outreach Ltd | Administrative & support | 2023 |
| Oort Cloud Holdings Limited | Financial services | 2024 |
| Open After Hours Ltd | Education | 2026 |
| Open on Limited | Information & communication | 2026 |
| Open Source Development Ltd | Information & communication | 2012 |
| Optarise Global Ltd | Administrative & support | 2025 |
| Orakai Ltd | Administrative & support | 2021 |
| Orange Henn Limited | Administrative & support | 2016 |
| Orbital Secure Limited | Information & communication | 2018 |
| Orchestra (Campton) Limited | Construction | 2018 |
| Orchestra (Colne) Ltd | Construction | 2026 |
| Orchestra (Commercial) Ltd | Construction | 2026 |
| Orchestra (Halstead) Ltd | Construction | 2025 |
| Orchestra (Helmdon) Limited | Construction | 2024 |
| Orchestra (Long Lane) Limited | Real estate | 2019 |
| Orchestra (Mount Hill) Ltd | Real estate | 2019 |
| Orchestra (Renhold) Limited | Construction | 2025 |
| Orchestra (Special Projects 1) Ltd | Construction | 2018 |
| Orchestra (Special Projects 2) Limited | Real estate | 2024 |
| Orchestra (ST Leonards) Ltd | Real estate | 2019 |
| Orchestra Homes Limited | Real estate | 2018 |
| Orchestra Land Holdings Ltd | Construction | 2024 |
| Orchestra Land Ltd | Professional & scientific | 2015 |
| Oreo Paragon Group Ltd | Financial services | 2023 |
| Orfords Property Maintenance Limited | Construction | 2008 |
| Orient Estates Limited | Real estate | 2006 |
| Orientt Limited | Information & communication | 2026 |
| Origination Technology Limited | Information & communication | 2010 |
| Orion Freehold Ltd | Real estate | 2013 |
| Oritz Solutions Ltd | Health & social care | 2019 |
| Orla Construction Ltd | Construction | 2022 |
| Orleh Limited | Wholesale & retail | 2025 |
| Orofos PA Limited | Real estate | 2021 |
| Ortakslar Ltd | Hospitality | 2023 |
| Orunmila Ltd | Professional & scientific | 2024 |
| OS1 Holdings Ltd | Financial services | 2026 |
| Osborne Gardens Ltd | Administrative & support | 2022 |
| Osirt Limited | Information & communication | 2022 |
| Osmani and Dutt Ltd | Construction | 2023 |
| Osprey Environmental Limited | Professional & scientific | 2017 |
| Osprey Remediation Ltd | Water & waste | 2021 |
| Osteocure Limited | Other services | 2017 |
| Osteopathic & Sports Therapy Limited | Other services | 2011 |
| Otto Building Ltd | Construction | 2024 |
| Ouardi Y M Limited | Other services | 2026 |
| OUR Properties Ltd | Real estate | 2018 |
| Outtray Co Ltd | Information & communication | 2025 |
| Ovel Ltd | Information & communication | 2025 |