| One Point Healthcare London Ltd | Public administration | 2025 |
| One Point Healthcare Ltd | Health & social care | 2022 |
| One Point Stationery Ltd | Wholesale & retail | 2021 |
| One Property Investment Ltd | Real estate | 2015 |
| One Sales Nation Ltd | Financial services | 2015 |
| One Sight Estates Limited | Real estate | 2013 |
| One Step Consultancy Ltd | Professional & scientific | 2022 |
| One Stop Allsorts Ltd | Wholesale & retail | 2025 |
| One Stop Care Services Limited | Health & social care | 2014 |
| One Stop Door Shop Ltd | Wholesale & retail | 2022 |
| One Stop Financial Planning Limited | Financial services | 2021 |
| One Stop Homes Ltd | Real estate | 2019 |
| One Tanna Ltd | Real estate | 2019 |
| One Three Eight Limited | Real estate | 2005 |
| One Time Investments Limited | Real estate | 2024 |
| One Vision Wealth Management Limited | Financial services | 2013 |
| One Window Holding Ltd | Wholesale & retail | 2024 |
| ONE2SHOP Ltd | Wholesale & retail | 2020 |
| Onebru Ltd | Wholesale & retail | 2025 |
| Onecare-Uk Ltd | Health & social care | 2012 |
| Oneforty Harrow Community Interest Company | Households | 2020 |
| Onelink Limited | Real estate | 1988 |
| Oneness Technologies Limited | Information & communication | 2008 |
| Onestop DN7 Ltd | Wholesale & retail | 2026 |
| Oneuplondon Ltd | Wholesale & retail | 2025 |
| Oneworld PVT Ltd | Information & communication | 2023 |
| Online Amnesia Ltd | Wholesale & retail | 2019 |
| Online Apps Group Ltd | Information & communication | 2015 |
| Online Curry Ltd | Hospitality | 2019 |
| Online Education Partnership Limited | Education | 2015 |
| Online Express Cars Ltd | Transport & logistics | 2011 |
| Online Trade Direct Ltd | Wholesale & retail | 2023 |
| Only 1 Kashmir Ltd | Hospitality | 2020 |
| Only 4 U Limited | Administrative & support | 2002 |
| Only Analytics Limited | Information & communication | 2018 |
| Only Care Limited | Health & social care | 2009 |
| Only Child Store Limited | Wholesale & retail | 2016 |
| Only Connect Partners Ltd | Arts & recreation | 2015 |
| Only Eggs Ltd | Hospitality | 2023 |
| Only Investments Ltd | Other services | 2010 |
| Only the Best Healthcare Solutions Ltd. | Health & social care | 2015 |
| ONLY1RENTALS Ltd | Wholesale & retail | 2021 |
| Ontheroof Agency Limited | Information & communication | 2023 |
| Ontheside Ltd | Arts & recreation | 2020 |
| Ontrack Engineering Limited | Administrative & support | 1997 |
| Ontrack Global Solutions Limited | Administrative & support | 2018 |
| Ontrack Management Solutions Ltd | Information & communication | 2006 |
| ONU Construction Ltd | Construction | 2026 |
| ONW Larry Ltd | Construction | 2025 |
| Onwa Limited | Professional & scientific | 2007 |
| Onwye LLP | | 2023 |
| Onyal Group Ltd | Construction | 2020 |
| Onyleo Ltd | Construction | 2026 |
| Onyx Realtors and Trading Consultants Ltd | Professional & scientific | 2025 |
| Oofy Prosser Limited | Professional & scientific | 2021 |
| Ookana Ltd | Professional & scientific | 2023 |
| Oorvi D UK Ltd | Wholesale & retail | 2019 |
| OP Residential Limited | Construction | 2025 |
| OPA Talent Limited | Administrative & support | 2023 |
| Opalscape Limited | Professional & scientific | 2012 |
| Opaque Development Limited | Construction | 2013 |
| OPC Solutions Limited | Wholesale & retail | 2006 |
| Opca Construction Ltd | Construction | 2020 |
| Open Answers Holdings Limited | Other services | 2022 |
| Open Car Doors Ltd | Wholesale & retail | 2018 |
| Open Consultants Ltd | Information & communication | 2010 |
| Open Doors Properties Ltd | Real estate | 2012 |
| Open Plan Group Ltd | Administrative & support | 2023 |
| Open Storage Group Ltd | Extraterritorial | 2024 |
| Open Ventures Ltd | Other services | 2023 |
| Open Wave Limited | Professional & scientific | 2024 |
| Openflex Limited | Professional & scientific | 2013 |
| Openhealth Clinic Limited | Other services | 2009 |
| Openpath AI CIC | Information & communication | 2025 |
| Opera & Alpha Tax Ltd | Professional & scientific | 2018 |
| Opera & Co Tax Limited | Extraterritorial | 2025 |
| Opera and Co UK Ltd | Professional & scientific | 2015 |
| Opera Tax Office Ltd | Professional & scientific | 2016 |
| Opes Capital Ltd | Financial services | 2023 |
| Opes Capital Properties Limited | Real estate | 2024 |
| Opes Evolutio Ltd | Professional & scientific | 2025 |
| Ophelia Dalston Ltd | Hospitality | 2014 |
| Ophir Road Residents Association Limited | Households | 2017 |
| Ophthalmic Medical and Surgical Ltd | Health & social care | 2012 |
| Ophthalmology Limited | Health & social care | 2019 |
| Opinion Catcher Ltd | Information & communication | 2012 |
| OPM Properties Services Limited | Real estate | 2002 |
| Opprima Limited | Wholesale & retail | 2020 |
| Optadian Limited | Information & communication | 2016 |
| Optical Dental Care Limited | Health & social care | 2011 |
| Optical Distribution Limited | Wholesale & retail | 2009 |
| Optima Care (Maidenhead) Ltd | Health & social care | 2010 |
| Optima Management Limited | Health & social care | 2009 |
| Optimise Financial Management Limited | Professional & scientific | 2012 |
| Optimise Resourcing Limited | Administrative & support | 2020 |
| Optimistics Limited | Professional & scientific | 2013 |
| Optimize Business Consulting Services Limited | Professional & scientific | 2015 |
| Optimize Wealth Ltd | Financial services | 2025 |
| Optimized Project Ltd | Construction | 2022 |
| Optimum Cleaners Ltd | Administrative & support | 2024 |