| One Nation Childcare Ltd | Health & social care | 2020 |
| One Nest Ltd | Wholesale & retail | 2025 |
| One Oak Motor Company Limited | Wholesale & retail | 2020 |
| One of One Motor Ltd | Wholesale & retail | 2025 |
| One Pentacle Limited | Financial services | 2019 |
| One Point Medical Services Limited | Health & social care | 2020 |
| One Race Ltd | Wholesale & retail | 2023 |
| One Real Estate Development Limited | Real estate | 2016 |
| One Residential Management (NE) Limited | Real estate | 2023 |
| One Stop GAS Ltd | Construction | 2023 |
| One Stop Motoring Services Ltd | Wholesale & retail | 2023 |
| One Stop Property Services NE Ltd | Other services | 2020 |
| One Time Gaming Ltd | Arts & recreation | 2020 |
| One Transport NE Limited | Administrative & support | 2023 |
| One Umbrella Commercial Cleaning FM Ltd | Administrative & support | 2025 |
| One Umrah Ltd | Administrative & support | 2023 |
| One Utility Bill Limited | Other services | 2015 |
| One Way TM Holdings Limited | Transport & logistics | 2019 |
| One World Ltd | Administrative & support | 2024 |
| ONE9 Health Consulting Ltd | Professional & scientific | 2020 |
| Onecardx Limited | Professional & scientific | 2023 |
| Oneclubsportswear Limited | Manufacturing | 2026 |
| Oneiros Ltd | Wholesale & retail | 2023 |
| Oneplusone Solutions Limited | Professional & scientific | 2015 |
| Onesmartclick Ltd | Hospitality | 2023 |
| Onester Games Ltd | Information & communication | 2025 |
| Onestowatchtalent Ltd | Professional & scientific | 2026 |
| Onetreeit Ltd | Information & communication | 2026 |
| Ongar MAC Limited | Professional & scientific | 2002 |
| Ongo Ltd | Transport & logistics | 2024 |
| Onion Heads Ltd | Information & communication | 2025 |
| Onjy Trade Ltd | Wholesale & retail | 2025 |
| Online Autoparts Limited | Wholesale & retail | 2023 |
| Online Engineering Limited | Manufacturing | 2006 |
| Online Grocery Store Ltd | Wholesale & retail | 2020 |
| Online Unified Maintenance Limited | Information & communication | 2009 |
| ONLINESHOPING728 Ltd | Wholesale & retail | 2025 |
| Only for YOU Content Limited | Professional & scientific | 2024 |
| Only Property Ltd | Real estate | 2022 |
| Only Sky Financial Solutions Limited | Other services | 2024 |
| Only Tanz NE Limited | Other services | 2026 |
| Only Waxing Ltd | Other services | 2020 |
| Onlydogsandhorsesnewcastle Ltd | Other services | 2020 |
| ONN Group Limited | Wholesale & retail | 2025 |
| Onsecondment Limited | Administrative & support | 2025 |
| Onside Collective Ltd | Professional & scientific | 2026 |
| Onsite Integrity Services Ltd | Manufacturing | 2025 |
| Onsite Metalwork Solutions Ltd | Manufacturing | 2025 |
| Onspec Optical Company Limited | Wholesale & retail | 2000 |
| Ontap Learning Ltd | Education | 2024 |
| Ontyne Plumbing & Heating Ltd | Construction | 2025 |
| Onuc Limited | Health & social care | 2015 |
| Onuco Ltd | Administrative & support | 2016 |
| Onward Trader Limited | Wholesale & retail | 2025 |
| Onyx Health Ltd | Administrative & support | 2003 |
| Oobe Group Limited | Administrative & support | 2016 |
| Oobe Ltd | Administrative & support | 2010 |
| OOH LAR LAR Ltd | Other services | 2023 |
| Ooiinc Ltd | Professional & scientific | 2025 |
| Oojaais Ltd | Information & communication | 2015 |
| Oolong Limited | Information & communication | 2021 |
| Ooma(Uk) Biotechnology Limited | Health & social care | 2023 |
| OP Solutions Ltd | Information & communication | 2026 |
| OPA Trading Limited | Professional & scientific | 2019 |
| Opal Access Limited | Construction | 2015 |
| Opapanax Ventures Ltd | Real estate | 2025 |
| Opci Limited | Information & communication | 2024 |
| Open Buck Consulting Limited | Professional & scientific | 2024 |
| Open Clasp Theatre Company | Arts & recreation | 2004 |
| Open Consentia Limited | Information & communication | 2025 |
| Open Engineering Solutions Ltd | Professional & scientific | 2023 |
| Open Hearth Hospitality Limited | Hospitality | 2026 |
| Open Spaces (UK) Limited | Wholesale & retail | 2012 |
| Open Wings Design Ltd | Manufacturing | 2018 |
| Opencast Software Europe Holdings Limited | Financial services | 2023 |
| Opencast Software Europe Limited | Information & communication | 2012 |
| Openlobbying CIC | Professional & scientific | 2026 |
| Openpredictor Holdings Ltd | Information & communication | 2024 |
| Operation Safedrop Limited | Public administration | 2024 |
| Operation Veteran CIC | Education | 2016 |
| Operations Support Services Limited | Professional & scientific | 2018 |
| Opes North East Limited | Real estate | 2024 |
| Opine Group Limited | Professional & scientific | 2015 |
| Opius Solutions Ltd | Construction | 2013 |
| Opportunity Global | Administrative & support | 2015 |
| Oppshop Limited | Wholesale & retail | 2022 |
| Opreatrans Limited | Transport & logistics | 2015 |
| OPS Intel Ltd | Professional & scientific | 2026 |
| Optical Suppliers Limited | Wholesale & retail | 2010 |
| Optima (Education) Limited | Education | 2014 |
| Optima Associates Limited | Information & communication | 2023 |
| Optimal Health Clinic NE Ltd | Health & social care | 2025 |
| Optimise Consulting Limited | Professional & scientific | 2022 |
| Optimise Payments Ltd | Wholesale & retail | 2025 |
| Optimise YOU Ltd | Health & social care | 2016 |
| Optimise Your AI Ltd | Wholesale & retail | 2025 |
| Optimise Your Energy Ltd | Construction | 2024 |
| Optimum Bots Ltd | Information & communication | 2024 |
| Optimum Setting OUT Services Ltd. | Construction | 2016 |
| Optimum Sports Limited | Wholesale & retail | 2019 |