| OFM4X4 UK Ltd | Wholesale & retail | 2022 |
| Ogakar Company Ltd | Professional & scientific | 2026 |
| Ognit Ltd | Information & communication | 2018 |
| OH YAH Limited | Information & communication | 2015 |
| OHD Media Ltd | Information & communication | 2018 |
| Ohemaa Limited | Professional & scientific | 2018 |
| OIF Properties Ltd | Real estate | 2024 |
| Oikogeneia Limited | Financial services | 2015 |
| Oilean Partners Limited | Professional & scientific | 2015 |
| Oilex Group LP | | 2017 |
| Ojas Consultancy Services Limited | Information & communication | 2010 |
| Ojuederie and Co Ltd | Transport & logistics | 2015 |
| OK Archive Ltd | Manufacturing | 2019 |
| Okla Properties Ltd | Real estate | 2026 |
| OKLEO17 LP | | 2018 |
| Old Farm Court (Bunns Lane) Management Company Limited | Real estate | 1979 |
| Old Oak WD Ltd | Construction | 2016 |
| Oldbridge Corporation Limited | Wholesale & retail | 1997 |
| Olden Investments Ltd | Real estate | 2024 |
| Olfsen Limited | Administrative & support | 2025 |
| Oliv International LP | | 2017 |
| Olive Garden Health Limited | Wholesale & retail | 2024 |
| Olive Grove MEP Consulting Limited | Professional & scientific | 2020 |
| Olive Lets Ltd | Real estate | 2025 |
| Olive Tree Baby Boutique Ltd | Wholesale & retail | 1996 |
| Oliveira & Co Limited | Other services | 2022 |
| Oliveira Brothers Services Ltd | Transport & logistics | 2014 |
| Oliver Bros. LLP | | 2010 |
| Oliver Cars Limited | Wholesale & retail | 2007 |
| Olivia HIL Limited | Real estate | 2017 |
| Olivios Bushey Ltd | Transport & logistics | 2010 |
| Olukat Limited | Administrative & support | 2008 |
| Oluwaferanmi Limited | Health & social care | 2010 |
| Olymp Academy Ltd | Education | 2025 |
| Olymp Group Ltd | Education | 2023 |
| Olymp Recruitment Services Ltd | Administrative & support | 2025 |
| Olympus Homes Ltd | Real estate | 2023 |
| OM Dynamics Ltd | Information & communication | 2008 |
| OM SAI Solutions Ltd | Information & communication | 2016 |
| OM Skanda Limited | Real estate | 2018 |
| OM Skandavale Ltd | Wholesale & retail | 2019 |
| OM Technologies Ltd | Professional & scientific | 2018 |
| Omaiz Life Coach Limited | Administrative & support | 2025 |
| OMC Technology Consulting Ltd | Information & communication | 2026 |
| Omeda Studios Limited | Information & communication | 2020 |
| Omega Electronics Limited | Extraterritorial | 1979 |
| Omega Engineering & Consultancy Group Limited | Professional & scientific | 2024 |
| Omega Import Export Limited | Wholesale & retail | 1970 |
| Omega Motorsport Limited | Wholesale & retail | 2001 |
| Omegatech Limited | Manufacturing | 1998 |
| Ometra Marketing Limited | Information & communication | 2023 |
| Omguru Digital Ltd | Information & communication | 2024 |
| OMJ Property Investment Limited | Real estate | 2018 |
| Omkar Estates Ltd | Real estate | 2025 |
| OMKSAI7 Ltd | Professional & scientific | 2026 |
| Omni Protection Limited | Information & communication | 2016 |
| Omnicroft Lettings Ltd | Real estate | 2026 |
| Omnisphere Limited | Wholesale & retail | 2023 |
| Omns Consulting Ltd | Professional & scientific | 2019 |
| Ompconsultancy Ltd | Professional & scientific | 2023 |
| Ompr Limited | Administrative & support | 2019 |
| OMS Mechanical & Electrical Limited | Construction | 2025 |
| Omyx Trading Limited | Other services | 2019 |
| ON and OFF Limited | Wholesale & retail | 2025 |
| ON OR Pause Ltd | Information & communication | 2006 |
| ON Road Limited | Real estate | 2024 |
| ON Screen Solutions Ltd | Information & communication | 2026 |
| On-Demand Plumbing & Heating Limited | Construction | 2020 |
| Once Amor CIC | Health & social care | 2023 |
| Once Arts Ltd | Arts & recreation | 2024 |
| Once Upon a Rescue Ltd | Agriculture & fishing | 2021 |
| Onconsult Limited | Professional & scientific | 2006 |
| One & Half Ltd | Other services | 2024 |
| One Agency Ltd | Professional & scientific | 2011 |
| One Billion Ltd | Information & communication | 2012 |
| One Click Accountant Ltd | Professional & scientific | 2014 |
| One Click Group UK Ltd | Financial services | 2015 |
| One Digital Tribe Ltd | Professional & scientific | 2023 |
| One Hundred New Cavendish Street Limited | Real estate | 1990 |
| One Markham Court & Corrigan Close Ltd | Households | 2018 |
| One Mountpark Management Limited | Households | 2025 |
| One Ring Ltd | Wholesale & retail | 2009 |
| One Systems Limited | Information & communication | 2006 |
| One Ymca | Hospitality | 2002 |
| One Ymca Development Company Ltd | Construction | 2018 |
| ONE6 Consulting and Management Limited | Administrative & support | 2009 |
| Oneil-Emsys Solutions Limited | Information & communication | 2025 |
| Oneiroworks Ltd | Information & communication | 2010 |
| Oneknotbarn Limited | Hospitality | 2026 |
| Onemango Limited | Information & communication | 2015 |
| Oneshot Build Ltd | Construction | 2019 |
| Onestop Financial Advisers Ltd | Financial services | 2023 |
| Onetick Sales Limited | Information & communication | 2018 |
| Onezeroone Technologies Ltd | Real estate | 2020 |
| Oniks Electronics Ltd | Wholesale & retail | 2017 |
| Online Depot Limited | Wholesale & retail | 2014 |
| Online Mattress Sale Ltd | Wholesale & retail | 2013 |
| Only Açai Ltd | Wholesale & retail | 2025 |
| Onlycardstcg Ltd | Wholesale & retail | 2024 |
| Onmain Limited | Information & communication | 2023 |