| Ocean Electronics & Security Limited | Administrative & support | 2014 |
| Octagon Nursery Ltd | Health & social care | 2018 |
| Octego Construction Ltd | Construction | 2016 |
| Octego Ltd | Construction | 2014 |
| Octego Projects Ltd | Construction | 2016 |
| Oculus Wealth Management (Medway) Limited | Financial services | 2014 |
| Odeon Care Limited | Health & social care | 2025 |
| Odisfire Ltd | Construction | 2025 |
| Odko Limited | Transport & logistics | 2023 |
| Odogwu Enterprise Ltd | Information & communication | 2025 |
| Odogwu Food Limited | Wholesale & retail | 2025 |
| Odott Consultancy Ltd | Professional & scientific | 2022 |
| ODR Scaffolding Ltd | Construction | 2023 |
| Odure Limited | Health & social care | 2018 |
| Odyssey Project Limited | Health & social care | 1995 |
| Ofad Ltd | Transport & logistics | 2024 |
| OFF Grid Excavations Ltd | Construction | 2021 |
| OFF Road Segway Sales UK Ltd | Wholesale & retail | 2023 |
| OFF White by Megs Ltd | Wholesale & retail | 2023 |
| Ognol UK Limited | Wholesale & retail | 2020 |
| Ognos Services Limited | Health & social care | 2023 |
| Oguine Frank Medicals Ltd | Health & social care | 2024 |
| Oh&S Services Limited | Professional & scientific | 2018 |
| Ohema Consulting Limited | Information & communication | 2004 |
| Ohmkara Limited | Hospitality | 2025 |
| Ohmkara Wellness Limited | Wholesale & retail | 2024 |
| OJ Construction Services Ltd | Construction | 2025 |
| OJ Strattech Limited | Information & communication | 2025 |
| Ojumo Ltd | Wholesale & retail | 2024 |
| Okoliane Investments Limited | Real estate | 2024 |
| Okoson Limited | Construction | 2018 |
| Oladokun Limited | Health & social care | 2023 |
| Olakem Ltd | Real estate | 2020 |
| Olamide Odunayo Limited | Information & communication | 2024 |
| Old Farm House Care Limited | Health & social care | 2003 |
| Old Farmhouse Healthcare Limited | Health & social care | 2017 |
| Old Fashion Values Limited | Construction | 2014 |
| Old Gate Social Care Consulting Limited | Health & social care | 2018 |
| Oldfield Services Limited | Professional & scientific | 2024 |
| Olean Beauty Ltd | Wholesale & retail | 2025 |
| Olimac Enterprise Ltd | Health & social care | 2022 |
| Olistone Limited | Wholesale & retail | 2014 |
| Olive 21 Limited | Professional & scientific | 2009 |
| Olive Branch Healthcare Limited | Health & social care | 2023 |
| Olive Innovations Ltd | Information & communication | 2014 |
| Oliver Harper Wealth Management Limited | Professional & scientific | 2022 |
| Olives Solicitors Limited | Professional & scientific | 2015 |
| Oliviay Limited | Wholesale & retail | 2022 |
| Ollystudio Limited | Professional & scientific | 2010 |
| Olstan Ltd | Information & communication | 2016 |
| Olu-Omo Limited | Professional & scientific | 2014 |
| Oluseunre Davies-Babatunde Cancer AID UK Limited | Wholesale & retail | 2025 |
| Olysia Ltd | Wholesale & retail | 2025 |
| OM Grab Services Ltd | Other services | 2019 |
| OM Lounge Ltd | Other services | 2020 |
| Omapaddy Limited | Administrative & support | 2019 |
| Omega Management Group Limited | Administrative & support | 2021 |
| Omega Records and Entertainment Ltd | Hospitality | 2025 |
| Omiomio TV Ltd | Information & communication | 2025 |
| Omnis Trading Ltd | Real estate | 2017 |
| Omnium Marine and Industrial Ltd | Transport & logistics | 2015 |
| Omnopia Ltd | Professional & scientific | 2026 |
| OMO Recruitment Ltd | Construction | 2026 |
| Omosco Investments Limited | Real estate | 2017 |
| ON Architecture London Limited | Professional & scientific | 2019 |
| ON Living Limited | Professional & scientific | 2016 |
| On-Weld Limited | Manufacturing | 2024 |
| On-X Properties Limited | Real estate | 2021 |
| ONA Capital Limited | Wholesale & retail | 2023 |
| One & Maike Ltd | Construction | 2021 |
| One Response International Ltd | Health & social care | 2024 |
| Onecore Ltd | Construction | 2025 |
| Oneplace Homes Ltd | Health & social care | 2021 |
| Onesavings Bank PLC | Financial services | 2010 |
| Onesixnine Design Ltd | Professional & scientific | 2023 |
| Online Corporate Services Ltd | Professional & scientific | 2024 |
| Onparhr Partners Ltd | Administrative & support | 2025 |
| Ooni Limited | Professional & scientific | 2012 |
| Open Rice Takeaway Ltd | Hospitality | 2024 |
| Open Spaces Property Ltd | Real estate | 2022 |
| Opison Property Limited | Real estate | 2023 |
| Oppio Limited | Manufacturing | 2020 |
| Optifin Ltd | Professional & scientific | 2025 |
| Optify Systems Ltd | Professional & scientific | 2024 |
| Optimised Synergy Ltd | Financial services | 2026 |
| Optimisteq Consulting Ltd | Information & communication | 2012 |
| Optimum Plus Care Ltd | Wholesale & retail | 2023 |
| Optimum Supported Housing Limited | Health & social care | 2017 |
| Option One Homes LLP | | 2023 |
| Opulence Capital Group Limited | Professional & scientific | 2025 |
| Opus Nominees Limited | Professional & scientific | 2008 |
| Opydej Ltd | Health & social care | 2026 |
| Oqidha Ltd | Professional & scientific | 2023 |
| Oraclemed Consulting Limited | Information & communication | 2014 |
| Orams A1 Distribution Limited | Transport & logistics | 2015 |
| Orangestone Assets Limited | Real estate | 2026 |
| Orangestone Logic Engineering Limited | Information & communication | 2026 |
| Orbem Capital Ltd | Professional & scientific | 2026 |
| Orchard Accountants Limited | Professional & scientific | 2002 |
| Orchard Accountants UK Limited | Professional & scientific | 2004 |