| On-Demand Marketing Ltd | Professional & scientific | 2021 |
| On-Point African Restaurant and Bar Ltd Ltd | Hospitality | 2021 |
| On-Top Skincare Limited | Wholesale & retail | 2018 |
| Onat Ltd. | Information & communication | 2014 |
| Ondes Property Limited | Real estate | 2023 |
| Ondes Wellbeing Ltd | Other services | 2024 |
| One Call (Nottingham) Limited | Real estate | 2021 |
| One Click Computing Limited | Information & communication | 2008 |
| One Earth Limited | Professional & scientific | 2025 |
| One Electric Limited | Construction | 2023 |
| One People Solutions Ltd | Professional & scientific | 2026 |
| One Quote Construction Ltd | Construction | 2024 |
| One Shah Square Ltd | Real estate | 2025 |
| One Step Mart Ltd | Wholesale & retail | 2024 |
| One Stop Atomic Ltd | Professional & scientific | 2023 |
| One Stop Car Sales Ltd | Wholesale & retail | 2023 |
| One Stop Car Solutions Ltd | Wholesale & retail | 2019 |
| One Stop Mini Market Ltd | Wholesale & retail | 2021 |
| One Stop Wellbeing Limited | Other services | 2020 |
| One UK Street Food Ltd | Hospitality | 2025 |
| One Utilities Limited | Energy | 2016 |
| One Utility Solutions Ltd | Construction | 2024 |
| One Vision Imaging Limited | Professional & scientific | 1984 |
| One-Dz Limited | Arts & recreation | 2012 |
| ONE10 Courier Limited | Other services | 2024 |
| ONE4RACING Ltd | Arts & recreation | 2026 |
| Onecote (Holdings) Limited | Hospitality | 2025 |
| Onepoint Events Limited | Hospitality | 2015 |
| Onestar Solutions Limited | Information & communication | 2015 |
| Onesystem Limited | Administrative & support | 2001 |
| Onetwozip Ltd | Wholesale & retail | 2025 |
| Oneunion Ltd | Hospitality | 2021 |
| Online Bedrooms Limited | Manufacturing | 2020 |
| Online Deals Hub Ltd | Wholesale & retail | 2024 |
| Online Models Ltd | Manufacturing | 2010 |
| Online Mortgage Solutions Limited | Financial services | 2022 |
| Online Personal Trainers Ltd | Health & social care | 2021 |
| Online Safety Supplies Limited | Extraterritorial | 2020 |
| Onlivio Ltd | Wholesale & retail | 2024 |
| Only Accountants Ltd | Professional & scientific | 2022 |
| Only Creator Management Limited | Professional & scientific | 2023 |
| Onomazo Limited | Professional & scientific | 2016 |
| Onomi Limited | Information & communication | 2014 |
| Onspark UK Limited | Information & communication | 2025 |
| Onyx (Eco) Ltd | Agriculture & fishing | 2026 |
| Onyx Group Corporation Ltd | Information & communication | 2024 |
| Onyx Platinum Transport Ltd | Transport & logistics | 2023 |
| Onyx Transport Ltd | Transport & logistics | 2021 |
| Ooyia Integrative Wellbeing Practices Limited | Health & social care | 2025 |
| Opal Accommodation Ltd | Hospitality | 2022 |
| Opal and Stone Properties Limited | Real estate | 2026 |
| OPC Muhammed BIN Umer Jaidi Ltd | Wholesale & retail | 2025 |
| Open Formula Limited | Information & communication | 2018 |
| Open Space Properties Ltd | Real estate | 2024 |
| Open Vision Events Ltd | Arts & recreation | 2023 |
| OPEN2LEARN Limited | Education | 2022 |
| Operations Excellence Consultants Ltd | Professional & scientific | 2023 |
| Operations Support Ltd | Professional & scientific | 2024 |
| Operatorsdirect Limited | Transport & logistics | 2017 |
| Opesbridge Ltd | Real estate | 2022 |
| Opex Business Systems Limited | Professional & scientific | 2017 |
| Opex Sports Limited | Administrative & support | 2022 |
| Ophthalmic Clarity - EYE Services Limited | Public administration | 2024 |
| Ophthalmic Locum Services Ltd | Extraterritorial | 2014 |
| Oppermann UK Limited | Professional & scientific | 1999 |
| Oppo Health UK Limited | Wholesale & retail | 2022 |
| Opportunity Knocks S&K Ltd | Health & social care | 2013 |
| Opsecpulse Ltd | Information & communication | 2025 |
| Opticare EYE Specialist Ltd | Health & social care | 2025 |
| Optik Leisure Limited | Construction | 2002 |
| Optik Properties Limited | Real estate | 2004 |
| Optimair Limited | Professional & scientific | 2000 |
| Optimal Care Agency Ltd | Administrative & support | 2022 |
| Optimal Plumbing and Heating Ltd | Construction | 2021 |
| Optimise Homes Ltd | Real estate | 2025 |
| Optimized Engineering Solutions Limited | Information & communication | 2012 |
| Optimum (Derby) Limited | Arts & recreation | 2021 |
| Optimum Temperature Solutions Ltd | Professional & scientific | 2019 |
| Optinest Eyecare Ltd | Wholesale & retail | 2025 |
| Option Estates Ltd | Real estate | 2024 |
| Option Finance Ltd | Professional & scientific | 2017 |
| Options & Choices Ltd | Transport & logistics | 2026 |
| Optique Vision (Derby) Limited | Wholesale & retail | 2006 |
| Optivis Engineering Consult Ltd | Professional & scientific | 2025 |
| Optix Express Ltd | Health & social care | 2022 |
| Opulent Motors Limited | Wholesale & retail | 2019 |
| Opulent Stays Ltd | Real estate | 2021 |
| Opus 67 Ltd | Professional & scientific | 2018 |
| Opus 67 Property Limited | Real estate | 2020 |
| Opus Contractors Limited | Construction | 2025 |
| Opus Contracts Limited | Information & communication | 1994 |
| Opus International Ltd | Professional & scientific | 2018 |
| Opusclean Limited | Administrative & support | 2008 |
| Opusclean Services Limited | Administrative & support | 2012 |
| ORA Weld Ltd | Manufacturing | 2018 |
| Orakle International Limited | Professional & scientific | 2026 |
| Orange Catering Ltd | Hospitality | 2026 |
| Orange Construction Derby Limited | Construction | 2014 |
| Orange Door Limited | Real estate | 2013 |
| Orange TIP Consulting Limited | Professional & scientific | 2017 |