| OMG Refresh Ltd | Wholesale & retail | 2026 |
| Omni FM Ltd | Other services | 2026 |
| Omnia Health Ltd | Financial services | 2023 |
| Omnifire Ltd | Professional & scientific | 2025 |
| Omnifort Technologies Limited | Information & communication | 2025 |
| Omnigroup Ltd | Financial services | 2025 |
| Omologato Consultancy Ltd | Wholesale & retail | 2012 |
| Omoyemeh Care Limited | Health & social care | 2019 |
| ON Air UK Limited | Administrative & support | 2009 |
| ON OFF Limited | Administrative & support | 2004 |
| ON Studios Ltd | Professional & scientific | 2015 |
| ON the Couch Psychotherapy Ltd | Health & social care | 2021 |
| ON Top Marketing Limited | Professional & scientific | 2020 |
| On-Site Worldwide Limited | Administrative & support | 2012 |
| Once Upon a Time Play Cafe Ltd | Hospitality | 2026 |
| One Big Solution Limited | Construction | 2020 |
| One East Ltd | Hospitality | 2019 |
| One Exchange Ltd | Real estate | 2015 |
| One Five Two Vehicle Conversions Limited | Manufacturing | 2016 |
| One MAN and HIS LOG Ltd | Agriculture & fishing | 2013 |
| One Stop Mortgages Limited | Financial services | 2024 |
| One Stop Picks Ltd | Wholesale & retail | 2025 |
| One Team Management Ltd | Professional & scientific | 2014 |
| One Tusk Limited | Health & social care | 2020 |
| One White Tree Ltd | Information & communication | 2026 |
| One Woman Consulting Ltd | Professional & scientific | 2011 |
| ONE2ONE Basketball Ltd | Administrative & support | 2017 |
| Onefourthree Consulting Ltd | Wholesale & retail | 2008 |
| Onereliable Ltd | Professional & scientific | 2024 |
| Oneview Property Management Ltd | Real estate | 2023 |
| ONH (Herts) Limited | Health & social care | 2013 |
| Onioninsight Ltd | Extraterritorial | 2023 |
| Onionplus (UK) Limited | Wholesale & retail | 2017 |
| Onlineporqupine Ltd | Manufacturing | 2026 |
| Only Halls and Smoke Ltd | Hospitality | 2025 |
| Only ME Uniq Ltd | Manufacturing | 2022 |
| Only Nans Limited | Information & communication | 2025 |
| Ono-Michi Ltd | Information & communication | 2026 |
| Onome Sankey Ltd | Professional & scientific | 2011 |
| Onset Film & TV Support Ltd | Administrative & support | 2025 |
| Onsite Lettings Limited | Real estate | 2010 |
| Onsite Property Limited | Real estate | 2016 |
| Ont-Uk Ltd | Health & social care | 2017 |
| Onthemicmedia Limited | Information & communication | 2014 |
| Onyx Electrical Services Ltd | Construction | 2021 |
| Onyx Gaming Limited | Information & communication | 2017 |
| Onyx Gaming Services Limited | Information & communication | 2025 |
| Onyx Gaming Technologies Limited | Information & communication | 2025 |
| Onyx Properties & Investments Ltd | Real estate | 2026 |
| Opal and Ivory Cleaning Co Ltd | Administrative & support | 2025 |
| Opal DOT Limited | Real estate | 2025 |
| Opal Lets Ltd | Real estate | 2026 |
| Opal Property & Renovation Ltd | Construction | 2026 |
| Opass Billings Wilson & Honey LLP | | 2008 |
| Opec Recruitment Ltd | Information & communication | 2024 |
| Open Plan Installations Ltd | Construction | 2021 |
| Open Plan Renovation Ltd | Construction | 2020 |
| Open Road Solutions Limited | Health & social care | 2013 |
| Open Road Visions | Health & social care | 1993 |
| Open Soul Space Ltd | Health & social care | 2023 |
| Openspace Advertising Limited | Professional & scientific | 2007 |
| Operum Ltd | Administrative & support | 2016 |
| Ophrys International Limited | Mining & quarrying | 2018 |
| Opra Management Services Limited | Households | 1998 |
| Oprica Ltd | Information & communication | 2015 |
| OPS Investments Ltd | Real estate | 2025 |
| Opstrive Holdings Limited | Administrative & support | 2026 |
| Opstrive Limited | Transport & logistics | 2026 |
| Optic Engineering Ltd | Other services | 2007 |
| Optic Fusion Limited | Information & communication | 2016 |
| Optic Matter Limited | Extraterritorial | 2017 |
| Optimal Digital Ltd | Professional & scientific | 2025 |
| Optimal Financial Services Limited | Financial services | 2008 |
| Optimise Construction Services Limited | Construction | 2012 |
| Optimised Male Ltd | Health & social care | 2023 |
| Optimize Financial Services Limited | Financial services | 2015 |
| Optimize Wheels Ltd | Wholesale & retail | 2016 |
| Optimum Architecture Limited | Professional & scientific | 2007 |
| Optimum Healthcare Services Limited | Transport & logistics | 2014 |
| Optimum Independent Financial Advisers Limited | Financial services | 2017 |
| Optimum Mental Health Consultants Ltd | Administrative & support | 2022 |
| Optimum Renovations Ltd | Extraterritorial | 2023 |
| Optimum S B M Services Ltd | Professional & scientific | 2024 |
| Optimus Maintenance Services Ltd | Construction | 2024 |
| Options UK Care Ltd | Health & social care | 2019 |
| Optymum Limited | Administrative & support | 1999 |
| Opulous Innovation Ltd. | Construction | 2013 |
| Opus 14 Limited | Arts & recreation | 2023 |
| Opus Engineering Ltd | Professional & scientific | 2025 |
| Opus Maintenance Services Ltd | Construction | 2022 |
| Opus Property Estates Limited | Real estate | 2024 |
| Opusfmgroup Limited | Administrative & support | 2020 |
| OR Productions Limited | Administrative & support | 2017 |
| OR Services Ltd | Health & social care | 2023 |
| Oracle International Transport Limited | Transport & logistics | 2026 |
| Oracle Property Services Limited | Real estate | 2001 |
| Orahey Limited | Health & social care | 2009 |
| Orange Penguin Limited | Wholesale & retail | 2023 |
| Orange PR and Marketing Ltd | Professional & scientific | 2024 |
| Oranje Homes Limited | Real estate | 2008 |