| Oceanix Technologies Limited | Information & communication | 2023 |
| Ochre Kitchen Ltd | Extraterritorial | 2021 |
| OCV Brickwork Ltd | Construction | 2026 |
| Odan Trading Limited | Wholesale & retail | 2025 |
| Odyssey Therapeutics UK Limited | Information & communication | 2018 |
| OFF Road Masters Limited | Education | 2001 |
| Og's Kitchen Limited | Hospitality | 2025 |
| Ohmage Electricals Ltd | Construction | 2025 |
| OIL Trade Ltd | Wholesale & retail | 2026 |
| Oilmycar Ltd | Wholesale & retail | 2025 |
| OIV Installing Ltd | Manufacturing | 2023 |
| Okobe Stephane Djegou Limited | Extraterritorial | 2022 |
| Old Normandy Consultancy Ltd | Construction | 2012 |
| Old Normandy Property Ltd | Real estate | 2021 |
| Oleptus Ltd | Information & communication | 2024 |
| OLF Holdings Company | Financial services | 1961 |
| OLI Rah’S Enterprise Ltd | Wholesale & retail | 2026 |
| Olisanctus Limited | Health & social care | 2022 |
| Oliver Carton Consulting Ltd | Information & communication | 2025 |
| Oliver Construction Ltd | Construction | 2024 |
| Olivia Ferrie Limited | Health & social care | 2026 |
| Ollie's Cloud Ltd | Extraterritorial | 2017 |
| Olly PYM Decorating Limited | Construction | 2018 |
| Omaton Ltd | Wholesale & retail | 2023 |
| Ombre Property Limited | Real estate | 2026 |
| Omega Canoes & Kayaks Ltd | Manufacturing | 2010 |
| Omni Engineering Ltd | Professional & scientific | 2024 |
| Omni Home Ltd | Construction | 2024 |
| Omnia Contracts Limited | Construction | 2019 |
| ON Point Groundworks Ltd | Administrative & support | 2022 |
| ON TAP Helmdon Ltd | Construction | 2010 |
| ON the Ball Learning Ltd | Information & communication | 2022 |
| ON the Edge Framing Limited | Wholesale & retail | 2022 |
| ON Track Plant Services Limited | Manufacturing | 2020 |
| Onaran Limited | Professional & scientific | 2006 |
| One Automotive Leasing Ltd | Administrative & support | 2025 |
| One Four Medical Ltd. | Health & social care | 2014 |
| One Future Media Limited | Information & communication | 2012 |
| One Money Ltd | Financial services | 2011 |
| One Sun Limited | Information & communication | 2014 |
| Onecall Services Ltd | Administrative & support | 2006 |
| Oneofone Ltd | Administrative & support | 2025 |
| Oneweigh Online Limited | Wholesale & retail | 2012 |
| Online Geeks Technologies Ltd | Information & communication | 2021 |
| Online Momentum Ltd | Professional & scientific | 2023 |
| Only Pans and Sauces Limited | Wholesale & retail | 2025 |
| Onlyyou Services Limited | Wholesale & retail | 2007 |
| Onpoint Sport Surfaces Ltd | Administrative & support | 2026 |
| OO Designs Limited | Information & communication | 1999 |
| Opal Systems Ltd | Other services | 2003 |
| Open Door Church (Kettering) | Other services | 2012 |
| Open Globe Recruitment Limited | Administrative & support | 2023 |
| Opening Corridors Ltd | Hospitality | 2025 |
| Openspaceservices Limited | Other services | 2002 |
| Opeyemi Osunsanmi Limited | Education | 2025 |
| Optical Suppliers Association Ltd. | Other services | 2002 |
| Opticare Innovations Ltd | Other services | 2024 |
| Optimill Consulting Services Limited | Professional & scientific | 2022 |
| Optimised Office Solutions Limited | Administrative & support | 2023 |
| Optimo Brick & Stone Limited | Construction | 2026 |
| OR Telecom Ltd | Information & communication | 2024 |
| Oracle Solutions Asbestos Limited | Construction | 2011 |
| Oracle Solutions Holdings Ltd | Other services | 2011 |
| Orange PIP Limited | Professional & scientific | 2000 |
| Orange Start Limited | Other services | 2023 |
| Oranges Plumbing and Heating Ltd | Construction | 2017 |
| Orbis Education Trust | Education | 2011 |
| Orbit Film & TV Limited | Information & communication | 2020 |
| Orchard Day Nursery Holdings Limited | Financial services | 2023 |
| Orchard Farm Trading Limited | Professional & scientific | 2013 |
| Order Funeral Coordination Ltd | Administrative & support | 2020 |
| Orient Search Limited | Administrative & support | 2023 |
| Oriental Diner Limited | Hospitality | 2014 |
| Oriental Traders Limited | Wholesale & retail | 2023 |
| Oris Plumbing and Heating Limited | Construction | 2023 |
| Oris Trading Limited | Extraterritorial | 2017 |
| Ortonomi Limited | Administrative & support | 2026 |
| Osborne Lodge Enterprises Ltd | Hospitality | 2020 |
| Oscar Commercial Ltd | Real estate | 2023 |
| Oscar Contractors Ltd | Construction | 2025 |
| Osenka Services Ltd | Administrative & support | 2025 |
| Osmeniguri Ltd | Hospitality | 2023 |
| Osmth England Limited | Education | 2025 |
| Osteo One Ltd | Health & social care | 2021 |
| Otto Offroad Ltd | Wholesale & retail | 2022 |
| Oundle Van MAN Limited | Other services | 2020 |
| OUR Lady's Catholic Club Community Interest Company | Hospitality | 2018 |
| Outram Tiling Ltd | Administrative & support | 2011 |
| Outside Inspiration Limited | Administrative & support | 2009 |
| Overcast Ltd | Financial services | 2026 |
| Overlay Agency Ltd | Wholesale & retail | 2023 |
| Overprint Systems Limited | Administrative & support | 2014 |
| Overstone Arms Ltd | Hospitality | 2018 |
| Overstone Road Development BA Limited | Construction | 2019 |
| OWL Stream Aviation Limited | Manufacturing | 2016 |
| Oxleys Electrical Ltd | Construction | 2021 |
| P & G Building Services (Kettering) Limited | Construction | 2010 |
| P & J Prime Holdings Ltd | Real estate | 2025 |
| P & M Sausages (London) Limited | Wholesale & retail | 2021 |
| P & M Sausages Limited | Manufacturing | 2021 |