| One Design & Planning Limited | Professional & scientific | 2016 |
| One Drop Timber Games Ltd. | Construction | 2024 |
| One Goal Football Academy Limited | Arts & recreation | 2023 |
| One Love in Christ Jesus Ministries | Other services | 2021 |
| One Time Consulting Ltd | Professional & scientific | 2026 |
| ONE4YOU Digital Ltd | Professional & scientific | 2022 |
| Ongar Town Football Club Ltd | Arts & recreation | 2025 |
| Online MD Ltd | Information & communication | 2020 |
| Onra& Ltd | Wholesale & retail | 2024 |
| Onur Property Limited | Real estate | 2017 |
| Onward Connections Limited | Health & social care | 2022 |
| Onyx Investment Advisory Ltd | Professional & scientific | 2025 |
| OOO Construction Ltd | Construction | 2024 |
| Oorg Ltd | Wholesale & retail | 2024 |
| Opaj Properties Limited | Real estate | 2026 |
| Opal Glow Beauty Ltd | Manufacturing | 2025 |
| Opecta Ltd | Construction | 2025 |
| Open Business Resources Ltd | Real estate | 2025 |
| Open Doors Technology Limited | Wholesale & retail | 2013 |
| Opochune Ltd | Information & communication | 2017 |
| Opted Properties Limited | Real estate | 2024 |
| Optic Services Ltd | Administrative & support | 2018 |
| Optim Construction Ltd | Construction | 2022 |
| Optimal Builders Limited | Construction | 2018 |
| Optimal Electrical Limited | Energy | 2020 |
| Optimal Energy Solutions Ltd | Energy | 2023 |
| Optimate AI Services Ltd | Information & communication | 2023 |
| Optimist Care Ltd | Health & social care | 2020 |
| Optimist Motors Ltd | Wholesale & retail | 2019 |
| Optimum Accounting & Finance Solutions Ltd | Professional & scientific | 2018 |
| Optimum Estate Management Ltd | Real estate | 2024 |
| Optimum Teeth Cleaning Limited | Health & social care | 2024 |
| Optimus MAX Ltd | Transport & logistics | 2017 |
| Or.Skin Ltd | Other services | 2022 |
| Orac NYE Limited | Health & social care | 2025 |
| Oracle Sports Consultants Ltd | Professional & scientific | 2015 |
| Orange Day Ltd | Transport & logistics | 2020 |
| Orbit Automation Ltd | Information & communication | 2025 |
| Orbit Networks Ltd | Information & communication | 2025 |
| Orbital Technology People Limited | Administrative & support | 2020 |
| Orbithinx Limited | Information & communication | 2025 |
| Orca Financials Limited | Administrative & support | 2024 |
| Orchard Garden Centre Limited | Wholesale & retail | 1979 |
| Ordained Limited | Other services | 2014 |
| ORI Care and Wellness Ltd | Other services | 2020 |
| Oriden Properties Limited | Real estate | 2025 |
| Oriental Supplies Ltd | Wholesale & retail | 2017 |
| Origin Direct Ltd | Manufacturing | 2019 |
| Original Alloys Limited | Wholesale & retail | 2022 |
| Origunloye Properties Ltd | Real estate | 2023 |
| Orion Ecom Limited | Wholesale & retail | 2025 |
| Ormi Healthcare Ltd | Health & social care | 2018 |
| Ormsby Support Services Limited | Professional & scientific | 2007 |
| Orontes Consulting Ltd | Professional & scientific | 2022 |
| Orpington Cakes Ltd | Wholesale & retail | 2024 |
| Orsett Estates Limited | Agriculture & fishing | 2008 |
| Orsett Golf Club Limited(The) | Arts & recreation | 1929 |
| Orsett Ltd | Professional & scientific | 2012 |
| Orsett Show Ground Limited | Real estate | 1983 |
| Orsett Storage Limited | Transport & logistics | 2021 |
| Orwell Place Management Company Limited | Real estate | 2006 |
| Osborne Commercial Assist Ltd | Wholesale & retail | 2022 |
| Osborne Construction & Property Management Ltd | Construction | 2023 |
| Oscar Building Services Ltd | Construction | 2015 |
| Oscar PAT UK Ltd. | Transport & logistics | 2022 |
| OSE Consultant Ltd | Information & communication | 2016 |
| Osheam Kitchen Limited | Hospitality | 2015 |
| Oskrlatino Producciones Limited | Hospitality | 2026 |
| OSO Gordo Freight Ltd | Transport & logistics | 2024 |
| Osor Rail Limited | Construction | 2017 |
| OSS Solutions Ltd | Construction | 2020 |
| Osshop Ltd | Wholesale & retail | 2022 |
| Otari. Ltd | Construction | 2017 |
| OTS Services Limited | Health & social care | 2025 |
| OUR Hair Ltd | Wholesale & retail | 2020 |
| OUR Travel Tips Limited | Information & communication | 2024 |
| Outsmarketing Limited | Administrative & support | 2011 |
| Outten Property Maintenance Limited | Construction | 2014 |
| OVB Trans Ltd | Transport & logistics | 2018 |
| Overcliff Transport Limited | Transport & logistics | 2020 |
| Oxar Ltd | Information & communication | 2024 |
| Oxford Home Study Centre Limited | Education | 2025 |
| Oxley Contracts Limited | Administrative & support | 2019 |
| Oxyeste Elite Living Ltd | Real estate | 2025 |
| Oxygen Factory Ltd | Wholesale & retail | 2025 |
| Oysterman Ltd | Wholesale & retail | 2019 |
| OZI27 Ltd | Hospitality | 2026 |
| Ozon Trans Limited | Transport & logistics | 2023 |
| P & T Howell Services Limited | Other services | 2015 |
| P C T Services (UK) Limited | Wholesale & retail | 2014 |
| P D Auto's Ltd | Wholesale & retail | 2026 |
| P D Consultants UK Limited | Administrative & support | 2008 |
| P Dervan & SON Limited | Construction | 2011 |
| P E P Limited | Professional & scientific | 1981 |
| P J Harper Services Ltd | Construction | 2016 |
| P M Construction Limited | Construction | 2010 |
| P Martin and SON Limited | Construction | 2012 |
| P O Property Company Limited | Households | 2004 |
| P&B Group Ltd | Construction | 2023 |
| P&D Future Homes Limited | Real estate | 2025 |