| OA Concepts Interiors Shop Ltd | Wholesale & retail | 2023 |
| OAC Capital Ltd | Professional & scientific | 2025 |
| Oak Lodge (Bursledon) Management Ltd | Households | 2024 |
| Oakbridge Skills Ltd | Education | 2026 |
| Oakencroft Limited | Construction | 2011 |
| Oakgreen Home Solutions Limited | Real estate | 2019 |
| Oakgreen Property Services Limited | Construction | 2022 |
| Oakhouse Interiors Southern Ltd | Construction | 2026 |
| Oakhurst Building Limited | Construction | 2023 |
| Oakmore Construction & Developments Ltd | Construction | 2023 |
| Oakwood Creations Limited | Construction | 2024 |
| Oasis of Hope Ltd | Administrative & support | 2022 |
| Obcit Limited | Information & communication | 2025 |
| Obliterated Ltd | Wholesale & retail | 2004 |
| Obliviar Events Limited | Arts & recreation | 2026 |
| Obliviar Limited | Arts & recreation | 2024 |
| Obrak Limited | Construction | 2025 |
| OC Roofing Contractors Ltd | Construction | 2022 |
| Ocean Marine Upholstery (Holdings) Ltd | Financial services | 2018 |
| Ocean Marine Upholstery Ltd | Manufacturing | 2015 |
| Ocean Medical Limited | Health & social care | 2023 |
| Oceanair Marine Limited | Extraterritorial | 2019 |
| Oceans Medical Ltd | Professional & scientific | 2022 |
| Oceanus Ltd | Construction | 2025 |
| Ocis Group Ltd | Manufacturing | 2025 |
| OCM Boxing Limited | Arts & recreation | 2024 |
| Ocourant Limited | Other services | 2010 |
| Oculitic Ltd | Information & communication | 2019 |
| Offshore Energy Inspections Ltd | Professional & scientific | 2021 |
| OH SO Lovely (London) Ltd | Education | 2017 |
| OIL Range Services Ltd | Construction | 2026 |
| Oj's Industrial Park (Portsmouth) LLP | | 2009 |
| OKO Carpentry Ltd | Construction | 2017 |
| Olanma Collection Ltd | Wholesale & retail | 2026 |
| Olar CM Ltd | Professional & scientific | 2022 |
| Old Drum Mews Management Company Limited | Households | 2014 |
| Old Farm Investments Limited | Real estate | 2024 |
| Old School Medical Services Limited | Health & social care | 2018 |
| Oldfield Air Conditioning Ltd | Construction | 2017 |
| Oldhamii Investment Group Limited | Financial services | 2022 |
| Oleonix Limited | Manufacturing | 1980 |
| Olivair (Home Improvements) Ltd | Construction | 2001 |
| Olivair (UK) Ltd | Financial services | 2001 |
| Olive and Sons Limited | Real estate | 2017 |
| Olive Tree House Interiors Limited | Wholesale & retail | 2022 |
| Oliver Bran Limited | Other services | 2019 |
| Oliver Keenan-Lyu Music Limited | Arts & recreation | 2023 |
| Oliver Walpole Ltd | Information & communication | 2026 |
| Oliver's Property Services Limited | Construction | 2015 |
| Oliver's Yard Cafe Ltd | Hospitality | 2011 |
| Olives Cafe Limited | Hospitality | 2022 |
| Ollievic Limited | Wholesale & retail | 2018 |
| Ollys Airport Cars Ltd | Transport & logistics | 2023 |
| Olof Medical Limited | Health & social care | 2019 |
| Olympus Tours Ltd | Administrative & support | 2024 |
| OMC Horseboxes Ltd | Manufacturing | 2025 |
| Omega Equine Limited | Wholesale & retail | 2017 |
| Omikron Investments Ltd | Construction | 2021 |
| Omni Sphere Group Ltd | Wholesale & retail | 2025 |
| Omni Talent Limited | Administrative & support | 2024 |
| Omniseal Ltd | Other services | 2018 |
| Omnisource 360 Ltd | Information & communication | 2020 |
| ON Budget Removals Limited | Transport & logistics | 2025 |
| ON Point Surveying Ltd | Construction | 2023 |
| ON Site Fabrication Services Ltd | Manufacturing | 2025 |
| ON Track Rail Limited | Construction | 2018 |
| ON Trend Interiors Ltd | Professional & scientific | 2025 |
| On-Site Workforce Limited | Transport & logistics | 2023 |
| One and Done (Carp Fishing) Ltd | Agriculture & fishing | 2026 |
| One Door Step Properties Limited | Real estate | 2025 |
| One Love Records Ltd | Wholesale & retail | 2026 |
| One Number Systems Ltd | Financial services | 2014 |
| One Punch Ltd | Arts & recreation | 2012 |
| One Step Move Ltd | Real estate | 2024 |
| One Way Resourcing Ltd | Administrative & support | 2004 |
| ONE567 Tech Limited | Information & communication | 2024 |
| Oneandtwo Limited | Real estate | 2024 |
| Onelaunch Ltd | Wholesale & retail | 2024 |
| Oneseventwo Limited | Administrative & support | 2023 |
| Onesta Associates Limited | Professional & scientific | 2013 |
| Onli Group Ltd | Administrative & support | 2021 |
| Online Safety UK Ltd | Education | 2018 |
| ONLY1 Logistics Limited | Transport & logistics | 2022 |
| ONLY1 Property Limited | Real estate | 2022 |
| Ontrack Training Ltd | Transport & logistics | 2024 |
| Onyx Agency Ltd | Professional & scientific | 2020 |
| OP Construction Ltd | Construction | 2018 |
| Opal Electrical Ltd | Construction | 2021 |
| Open Horizons Ltd | Administrative & support | 2024 |
| Open Space Marketing Limited | Administrative & support | 2024 |
| Opentill Limited | Professional & scientific | 2024 |
| Operating Services Limited | Construction | 2018 |
| Operator Compliance & Risk Services Ltd | Professional & scientific | 2024 |
| Opsols Ltd | Professional & scientific | 2022 |
| Opswell Ltd | Information & communication | 2024 |
| Optima Commercial Limited | Professional & scientific | 2025 |
| Optima Consulting Limited | Professional & scientific | 2018 |
| Optimum in Vita Holdings Ltd | Financial services | 2023 |
| Optivalve Consultants Ltd | Professional & scientific | 2025 |
| Optus Digital Ltd | Professional & scientific | 2014 |