| Onmain Systems Limited | Wholesale & retail | 2024 |
| Onmart Limited | Wholesale & retail | 2024 |
| Onstage Travel Limited | Administrative & support | 2019 |
| ONT Contractor Limited | Construction | 2019 |
| Onthecloud IT Ltd | Information & communication | 2013 |
| Onwheel Catering Limited | Hospitality | 2025 |
| Onye Ltd | Information & communication | 2024 |
| Onyoursite Ltd | Other services | 2025 |
| Onyx Pest Services Ltd | Administrative & support | 2022 |
| Onyxfusion Ltd | Administrative & support | 2021 |
| OOH Marketing Ltd | Professional & scientific | 2025 |
| OOM Real Estate Limited | Real estate | 2022 |
| Ootd London Ltd | Wholesale & retail | 2025 |
| Opal Associates Limited | Professional & scientific | 2025 |
| Oparo Social Chatsworth Ltd | Real estate | 2021 |
| Opazlight International Limited | Wholesale & retail | 2004 |
| OPC Software Ltd | Information & communication | 2025 |
| Open Care Ltd | Health & social care | 2023 |
| Open Cognition Ltd | Other services | 2023 |
| Open Solutions Resource Ltd | Extraterritorial | 2014 |
| Opendelta Limited | Education | 2021 |
| Opentrade Ltd | Real estate | 2021 |
| Operating Partners Private Equity Limited | Professional & scientific | 2016 |
| Operations Alchemist Ltd | Professional & scientific | 2023 |
| Operoris Limited | Information & communication | 2014 |
| Opes Advisory Ltd | Professional & scientific | 2026 |
| Opportunity Accounts Ltd | Professional & scientific | 2024 |
| Optical Thinking Limited | Administrative & support | 2017 |
| Opticalis Limited | Health & social care | 2017 |
| Opticool Services Ltd | Manufacturing | 2022 |
| Optifiber Connect Ltd | Information & communication | 2023 |
| Optimal Business Management Ltd | Professional & scientific | 2022 |
| Optimal Compliance Services (London) LLP | | 2015 |
| Optimal Health Goals Ltd | Manufacturing | 2025 |
| Optimis Tech Ltd | Administrative & support | 2025 |
| Optimistic Goods Limited | Wholesale & retail | 2025 |
| Optimus PCB Ltd | Administrative & support | 2013 |
| Optimus Realty Group Ltd | Real estate | 2026 |
| Optonica LED UK Ltd | Wholesale & retail | 2015 |
| Optus UK Limited | Information & communication | 2024 |
| Opulent Stationery Ltd | Manufacturing | 2024 |
| Opulentfleet Ltd | Wholesale & retail | 2024 |
| Opus Infra Ltd | Real estate | 2014 |
| Opus People Limited | Administrative & support | 2019 |
| Opus Race Promotions Ltd | Arts & recreation | 2025 |
| Ora-Tech (UK) Limited | Construction | 2009 |
| Oracle Kitchens Ltd | Information & communication | 2023 |
| Oracon Limited | Financial services | 2009 |
| Oradi Ltd | Health & social care | 2006 |
| Orafusion IT Ltd. | Information & communication | 2019 |
| Oral Care Solutions Limited | Health & social care | 2012 |
| Oramaa Lawrence Architects Limited | Professional & scientific | 2011 |
| Orange Moon Properties Limited | Real estate | 2023 |
| Oranpharma Ltd | Professional & scientific | 2020 |
| Orbital Fasteners Holdings Limited | Financial services | 2012 |
| Orbital Fasteners Limited | Wholesale & retail | 1976 |
| Orbital Haulageuk Limited | Transport & logistics | 2026 |
| Orbital Signs Limited | Wholesale & retail | 1998 |
| Orbitalroofing&Scaffolding Limited | Construction | 2018 |
| Orbitz Consulting Limited | Financial services | 2015 |
| Orchard Classic Decals Limited | Professional & scientific | 2022 |
| Orchard Properties Services Limited | Real estate | 2020 |
| Orchestra Capital LLP | | 2013 |
| Orchestrate Marketing Limited | Professional & scientific | 2019 |
| Orchid House Properties Limited | Real estate | 1995 |
| Ordell Consulting Limited | Information & communication | 2007 |
| Orderly Disruption Limited | Professional & scientific | 2014 |
| Ordermile Limited | Real estate | 1987 |
| Oreker Homes Ltd | Real estate | 1997 |
| ORG Electrical Ltd | Construction | 2025 |
| Organic Accounting Solutions Ltd | Professional & scientific | 2020 |
| Organicano Ltd | Hospitality | 2025 |
| Organization for New Occupational Opportunities C.I.C. | Education | 2019 |
| Orhan IT Services Ltd. | Information & communication | 2016 |
| Orient Solutions Limited | Wholesale & retail | 2003 |
| Origin TEN Ltd | Information & communication | 2021 |
| Original Comms Ltd | Professional & scientific | 2021 |
| Original Home Pizza Ltd | Hospitality | 2020 |
| Original Mind Limited | Health & social care | 2026 |
| Original Paving Ltd | Construction | 2025 |
| Original Vimco Foods Ltd | Wholesale & retail | 2024 |
| Originalskin Ltd | Wholesale & retail | 2024 |
| Origins Mcma Hub Limited | Arts & recreation | 2019 |
| Orion Coaching & Consulting Limited | Wholesale & retail | 2016 |
| Orion UK Business Solutions Limited | Professional & scientific | 2026 |
| Orique Ltd | Administrative & support | 2024 |
| Orix Instrumentation UK Limited | Administrative & support | 2025 |
| Ormos Trades Limited | Wholesale & retail | 2012 |
| Ornado Medicine LLP | | 2013 |
| Orpman Limited | Households | 2009 |
| ORS Legal Services Ltd | Professional & scientific | 2025 |
| Orwell Cleaning Limited | Administrative & support | 2021 |
| Osazee & Robo's Homes Limited | Real estate | 2026 |
| Osborn & Brown Limited | | 1990 |
| Osbourne Therapy Limited | Health & social care | 2013 |
| Oshwal Properties Limited | Real estate | 2004 |
| OSI Consultancy Ltd | Professional & scientific | 2022 |
| Osib Limited | Professional & scientific | 2010 |
| OSP Social Ltd | Arts & recreation | 2025 |
| Osprey Engines Ltd | Manufacturing | 2013 |