| Onboardbusiness Limited | Wholesale & retail | 2025 |
| Once Upon a Time 235 Coffee Ltd | Hospitality | 2025 |
| One Base Construction Logistics Ltd | Construction | 2025 |
| One Car Planet Ltd | Wholesale & retail | 2020 |
| One Casa Ltd | Real estate | 2021 |
| One Click GET a Way Limited | Wholesale & retail | 2024 |
| One Day at a Time Courier Ltd | Transport & logistics | 2021 |
| One Health Check Ltd | Health & social care | 2025 |
| One Heart Group Ltd | Real estate | 2025 |
| One Hub Marketing Limited | Information & communication | 2019 |
| One Hundred Coffee Ltd | Hospitality | 2019 |
| One JG Limited | Wholesale & retail | 2022 |
| One NB Ltd | Real estate | 2021 |
| One Pillar Properties Ltd | Real estate | 2025 |
| One Place East Ltd | Health & social care | 1995 |
| One Revenue Limited | Administrative & support | 2020 |
| One Shop Spot Stop Ltd | Wholesale & retail | 2025 |
| One Stop Building Services (London) Ltd | Construction | 2021 |
| One Stop Food and Wine Convenience Limited | Wholesale & retail | 2018 |
| One Stop Kitchenware Ltd | Wholesale & retail | 2015 |
| One Stop Property Services (London) Ltd | Other services | 2020 |
| One Stop Services London Ltd | Transport & logistics | 2023 |
| One Telos Ltd | Wholesale & retail | 2021 |
| One TEN Traders Ltd | Wholesale & retail | 2022 |
| One Way Estates Ltd | Real estate | 2019 |
| One WOK Ltd | Hospitality | 2025 |
| Onebyme Limited | Manufacturing | 2016 |
| Oneness Smiles Ltd | Health & social care | 2025 |
| Onesodas Limited | Administrative & support | 2023 |
| Onestep Cart Ltd | Wholesale & retail | 2025 |
| Onestep Management Service Ltd | Professional & scientific | 2026 |
| Oneten Online Stores Ltd | Wholesale & retail | 2025 |
| ONETO10 Ltd | Wholesale & retail | 2025 |
| Onik Medical Ltd | Health & social care | 2025 |
| Onin Capital Investment Ltd | Professional & scientific | 2026 |
| Online and Instore Furniture Ltd | Wholesale & retail | 2024 |
| Online Business College Ltd | Education | 2024 |
| Online Filing Solutions Limited | Extraterritorial | 2014 |
| Online NRI Limited | Information & communication | 2024 |
| Online Quran in the World . Ltd | Education | 2025 |
| Online Shower Panels Ltd | Wholesale & retail | 2021 |
| Only Branded Limited | Wholesale & retail | 2017 |
| Only for YOU Holidays Ltd | Administrative & support | 2025 |
| Only Seconds Limited | Professional & scientific | 2018 |
| Onlygraded Meta Ltd | Information & communication | 2024 |
| Onmadrive Ltd | Wholesale & retail | 2020 |
| Onpoint Strategies Ltd | Manufacturing | 2016 |
| Onsen Limited | Wholesale & retail | 2020 |
| Ontogenesis Limited | Information & communication | 2009 |
| Ontoversial Ltd | Information & communication | 2026 |
| Onur BBQ Ltd | Hospitality | 2024 |
| Onvila Ltd | Construction | 2024 |
| Onyii's Cosmetics Ltd | Manufacturing | 2019 |
| Onyx Future Ltd | Real estate | 2016 |
| Onyx Leisure Limited | Mining & quarrying | 2019 |
| OP Sealants Ltd | Construction | 2024 |
| OPA Services Ltd | Transport & logistics | 2019 |
| Opal & Ivory Ltd | Health & social care | 2025 |
| Oparo Social Langley Ltd | Real estate | 2021 |
| Open Cloud Security Ltd | Administrative & support | 2025 |
| Open DOT London Ltd | Hospitality | 2026 |
| Open Hearts Foundation Ltd | Health & social care | 2024 |
| Open Key Properties Limited | Real estate | 2026 |
| Open Reach Solutions Ltd | Health & social care | 2026 |
| Open Road Couriers Ltd | Transport & logistics | 2025 |
| OPEN22 Solutions Limited | Professional & scientific | 2023 |
| Openbuy Global E-Commerce Co., Ltd | Wholesale & retail | 2024 |
| Openskies Cloud Solutions Limited | Information & communication | 2014 |
| Opentec Ltd | Professional & scientific | 2015 |
| Opentech Solutions Ltd | Information & communication | 2012 |
| Opex M Ltd | Wholesale & retail | 2023 |
| Opex Property Management Ltd | Real estate | 2022 |
| Ophthalmic Sound Solutions Ltd | Health & social care | 2019 |
| OPM Real Estate Ltd | Real estate | 2010 |
| Oppo Investment Ltd | Real estate | 2025 |
| Oppurtunity Limited | Real estate | 2020 |
| Opsat Shipping Limited | Transport & logistics | 2015 |
| OPT & Investments Ltd | Real estate | 2023 |
| Opticient Ltd | Information & communication | 2016 |
| Optikit Limited | Wholesale & retail | 2018 |
| Optilocum Ltd | Health & social care | 2015 |
| Optilox Ltd | Health & social care | 2015 |
| Optim SEO Ltd | Information & communication | 2021 |
| Optima Facilities Limited | Administrative & support | 2025 |
| Optima Stays Ltd | Real estate | 2025 |
| Optima Ventures Limited | Transport & logistics | 2026 |
| Optimal Care Solutions Ltd | Health & social care | 2023 |
| Optimal Refurb Solutions Ltd | Professional & scientific | 2016 |
| Optimal Security Solutions Limited | Administrative & support | 2024 |
| Optimal Transform Ltd | Information & communication | 2021 |
| Optimi Global Ltd | Professional & scientific | 2023 |
| Optimize Solar Solutions Ltd | Extraterritorial | 2015 |
| Optimon Locum Services Limited | Health & social care | 2022 |
| Optimum Cleaning Co. Ltd | Professional & scientific | 2023 |
| Optimum Deals Ltd | Wholesale & retail | 2024 |
| Optimum GAS Energy Ltd | Construction | 2019 |
| Optimum Heat Ltd | Construction | 2021 |
| Optimum Merchandisers Ltd | Wholesale & retail | 2016 |
| Optimyx Ltd | Wholesale & retail | 2024 |
| Optiserv Ltd | Health & social care | 2022 |