| O J Engineering Limited | Other services | 2014 |
| O&E Solutions Ltd | Construction | 2026 |
| O. Dunne Ltd | Construction | 2024 |
| OAA Consultants Limited | Professional & scientific | 2019 |
| OAB Enterprise Solutions Ltd | Information & communication | 2026 |
| Oak Group Accounting Services Limited | Professional & scientific | 2025 |
| Oak Management Services Ltd | Professional & scientific | 2023 |
| Oakbridge Services Ltd | Health & social care | 2023 |
| Oakden Holdings Limited | Financial services | 2020 |
| Oakden Property Developments Limited | Construction | 2021 |
| OAL Consultancy Ltd | Professional & scientific | 2020 |
| Oandsen Ltd | Construction | 2014 |
| OBAJ1 Ltd | Information & communication | 2017 |
| Obazda Limited | Administrative & support | 2006 |
| Occasio Partners Ltd | Professional & scientific | 2024 |
| Occasio Solutions Ltd | Information & communication | 2024 |
| Ochre Red Limited | Information & communication | 2013 |
| Octobyte Limited | Public administration | 2019 |
| Odyssey Health Club & Spa Limited | Arts & recreation | 2019 |
| Odyssey Knebworth Limited | Health & social care | 2000 |
| Oesophix Healthcare Limited | Health & social care | 2026 |
| OFF Limits Entertainment Limited | Professional & scientific | 2017 |
| OFF Limits Holdings SPV Limited | Financial services | 2023 |
| OFF Limits Sport Ltd | Other services | 2021 |
| Offeraptor Ltd | Wholesale & retail | 2026 |
| Officeduster Ltd | Administrative & support | 2024 |
| OG Hardware UK Ltd | Wholesale & retail | 2026 |
| OG Products Ltd | Wholesale & retail | 2024 |
| Ogel IT Ltd | Information & communication | 2016 |
| Ogori Restaurant Ltd | Hospitality | 2024 |
| Ogunko Gokey Properties Limited | Real estate | 2024 |
| OJJ Cleaning Ltd | Administrative & support | 2022 |
| OK to BE CIC | Education | 2023 |
| Olamazi Ltd | Wholesale & retail | 2022 |
| Old Street Marketing Limited | Professional & scientific | 2014 |
| Old Town Nail Bar Ltd | Other services | 2019 |
| Old Town Vape Bar Ltd | Wholesale & retail | 2018 |
| Old Welwyn MOT Centre Ltd | Wholesale & retail | 2018 |
| Olfactive London Ltd | Manufacturing | 2017 |
| Olive and Daisy Limited | Arts & recreation | 2010 |
| Olive Promotions Limited | Other services | 2016 |
| Olive Tree Properties Nationwide Limited | Other services | 2018 |
| Oliver Duffett Limited | Other services | 2023 |
| Olivialee Properties Limited | Real estate | 2024 |
| Olufunmi Ltd | Administrative & support | 2020 |
| Olympus Executive Driver Services Ltd | Transport & logistics | 2026 |
| Olympus Midco Limited | Financial services | 2018 |
| OLYV8 Consultancy Ltd | Professional & scientific | 2025 |
| OM Electrical Services Ltd | Construction | 2015 |
| OM Investment Group Ltd | Real estate | 2026 |
| Omba First Line Services Ltd | Public administration | 2023 |
| Ombra Design Limited | Information & communication | 2017 |
| Omega Citylifts Installations Limited | Construction | 1993 |
| Omega Citylifts Services Limited | Manufacturing | 1996 |
| Omega Commercial Solutions Limited | Financial services | 2005 |
| Omega Elevators Limited | Extraterritorial | 2008 |
| Omega Financial Services Limited | Financial services | 2004 |
| Omega Invoice Finance Solutions Limited | Financial services | 2012 |
| Omegha M. Ltd | Hospitality | 2017 |
| Omnia AV Ltd | Financial services | 2026 |
| Omnia Property Management Group Ltd | Real estate | 2024 |
| Omnia TV Ltd | Financial services | 2026 |
| Omniledger Holdings Limited | Financial services | 2024 |
| Omniledger Limited | Information & communication | 1982 |
| Omnis Intel Holdings Limited | Financial services | 2026 |
| Omnitech Capital Limited | Financial services | 2022 |
| Omnium Leisure Ltd | Hospitality | 2021 |
| Omosoga Consulting Ltd | Information & communication | 2022 |
| Omrani Limited | Other services | 2002 |
| ON the Green Ltd. | Hospitality | 2015 |
| ON the Run Developments Limited | Real estate | 2021 |
| ON the Run Properties Limited | Real estate | 2020 |
| One Acre Developments Ltd | Construction | 2025 |
| One Call Property Solutions Ltd | Other services | 2024 |
| One Facility Limited | Real estate | 2005 |
| One Law for ALL | Other services | 2012 |
| One Xperience Consultancy Ltd | Other services | 2021 |
| One Zero Electrical Ltd | Construction | 2024 |
| ONE28 Ltd | Manufacturing | 2021 |
| ONE4EM Ltd | Construction | 2025 |
| Onitac Services Limited | Other services | 2021 |
| Onkos Bioservices Ltd | Professional & scientific | 2019 |
| Only Losers WIN Happily Ltd | Information & communication | 2018 |
| Onlyairsoft Ltd | Wholesale & retail | 2016 |
| OO8 Business and Office Solutions Ltd | Professional & scientific | 2026 |
| OP Hounslow Ltd | Real estate | 2019 |
| Op-Bot Ltd | Professional & scientific | 2024 |
| Opabid Datatech Limited | Information & communication | 2024 |
| Opar Group Limited | Construction | 2018 |
| Oporo Asia Limited | Public administration | 2024 |
| OPS Elevate Limited | Information & communication | 2024 |
| Optihold Limited | Other services | 2025 |
| Optimization Limited | Professional & scientific | 2012 |
| Optimum Control Engineering Ltd | Manufacturing | 2023 |
| Optimum Design & Development Ltd | Wholesale & retail | 2016 |
| Optimum Interiors Limited | Other services | 2019 |
| Optimus Specialist Contractors Group Ltd | Construction | 2025 |
| Option B Logistics Limited | Transport & logistics | 2026 |
| Options Health Care Services Limited | Health & social care | 2011 |
| Options Holding Limited | Financial services | 2023 |