| Olympus Power Holding Company Ltd | Financial services | 2018 |
| Olympus Power Ltd | Energy | 2015 |
| Olympus Solar Holdings Ltd | Energy | 2020 |
| Olympus Solar I Ltd | Energy | 2021 |
| Olympus Solar III Ltd | Energy | 2024 |
| Olympus Solar IV Ltd | Energy | 2024 |
| Olympus Solar V Ltd | Energy | 2024 |
| Olympus Solar VI Ltd | Energy | 2024 |
| Omgag Ltd | Information & communication | 2025 |
| Omi.Jd.Co Ltd | Wholesale & retail | 2022 |
| ON PAR Properties Ltd | Real estate | 2023 |
| ON the Hill C.I.C. | Administrative & support | 2017 |
| ON the Pitch Coaching Ltd | Professional & scientific | 2018 |
| ON the Town Limited | Real estate | 2016 |
| ON Track Support Ltd | Professional & scientific | 2021 |
| ON White Horses Ltd | Professional & scientific | 2022 |
| Onar Recruitment Ltd | Administrative & support | 2019 |
| One 2 One Fitness and Lifestyle Limited | Arts & recreation | 2020 |
| One Belmont Road Limited | Households | 2026 |
| One Bright Spark Limited | Manufacturing | 2011 |
| One Grosvenor Place Management Company Limited | Households | 1991 |
| One Hunter Limited | Information & communication | 2015 |
| One Spine Ltd | Administrative & support | 2015 |
| One Tamar Limited | Financial services | 2021 |
| One Thousand Stars Ltd | Hospitality | 2022 |
| One Twenty A2 Ltd | Manufacturing | 2020 |
| One Voice Holdings Limited | Financial services | 2021 |
| One Voice Media & PR Limited | Professional & scientific | 1997 |
| Onelife Training UK Limited | Education | 2008 |
| Oneserve Group Limited | Financial services | 2015 |
| Oneserve Ltd | Information & communication | 2010 |
| Oneswifty Ltd | Information & communication | 2020 |
| Oneten Realty Ltd | Real estate | 2024 |
| Online Safety Centre Limited | Information & communication | 2026 |
| Onlyvansmechanical Ltd | Wholesale & retail | 2024 |
| ONR Luxury Chauffeurs Ltd | Transport & logistics | 2025 |
| ONT Electrical Contractors Limited | Construction | 2021 |
| Ontra Capital Limited | Professional & scientific | 2017 |
| Onuk Ltd | Real estate | 2023 |
| Onvo Tech Ltd | Information & communication | 2026 |
| Onyali Limited | Hospitality | 2021 |
| Onyex Ltd | Real estate | 2017 |
| Onyx (ESP 02) Management Company Limited | Households | 2024 |
| Onyx Business Parks Barnstaple Ltd | Construction | 2022 |
| Onyx Business Parks Exeter Ltd | Construction | 2023 |
| Onyx Business Parks Ltd | Financial services | 2020 |
| Onyx Business Parks Tolvaddon Ltd | Construction | 2023 |
| Onyx Business Parks Wellington Ltd | Construction | 2022 |
| Onyx Tolvaddon Management Company Ltd | Households | 2026 |
| OON Skin Clinic Limited | Health & social care | 2023 |
| OP1 Construction Ltd | Construction | 2021 |
| OPASOUTH1 Ltd | Hospitality | 2026 |
| Opcd Limited | Health & social care | 2010 |
| Open Doors Ltd | Real estate | 2021 |
| Open Doors Property Management Ltd | Real estate | 2022 |
| Open Epoch Equality Ltd | Professional & scientific | 2025 |
| Open Minds Wellbeing Ltd | Education | 2019 |
| Open Salotto CIC | Hospitality | 2026 |
| Openeye Consulting Limited | Professional & scientific | 2005 |
| Openoffer Limited | Extraterritorial | 2009 |
| Openrain Limited | Administrative & support | 2025 |
| Operation Good Guys Productions Limited | Information & communication | 2016 |
| Operator Tickets Ltd | Education | 2007 |
| Opie & Associates Ltd | Financial services | 2018 |
| Opmal Limited | Real estate | 2011 |
| Opportunity Lettings Ltd | Real estate | 2019 |
| Opticab Ltd | Information & communication | 2025 |
| Optimal Finance Solutions Ltd | Information & communication | 2022 |
| Optimum Links Limited | Administrative & support | 1996 |
| Optimum Performance (SW) Ltd | Arts & recreation | 2025 |
| Optimus Navis Consultants Limited | Other services | 2017 |
| Optimus Navis Limited | Transport & logistics | 2008 |
| Optionallied Limited | Real estate | 1997 |
| Optix Solutions Ltd | Information & communication | 1999 |
| Optix Ventures Limited | Information & communication | 2014 |
| Opus Group (UK) Limited | Professional & scientific | 2011 |
| Opus Minus Limited | Information & communication | 2018 |
| Opus Plastics Limited | Manufacturing | 2009 |
| Oqira Ltd | Professional & scientific | 2025 |
| OR and Worley Family Partnership LLP | | 2022 |
| Orange Tree Trading (UK) Limited | Wholesale & retail | 1998 |
| Orangutan TM Limited | Professional & scientific | 2001 |
| Orbis Consulting Engineers Limited | Professional & scientific | 2013 |
| Orchard Cosmetic Limited | Health & social care | 2011 |
| Orchard Cottage Lettings Limited | Real estate | 2025 |
| Orchard Court (Silverton) Management Limited | Households | 2018 |
| Orchard Court Broadway Management Company Limited | Households | 2019 |
| Orchard House & Shrublands (Management) Limited | Households | 1981 |
| Orchard House Rentals Ltd | Real estate | 2024 |
| Orchard Ledgers Ltd | Professional & scientific | 2025 |
| Orchard View Devon Ltd | Wholesale & retail | 2025 |
| Orchard Way (Topsham) Management Limited | Real estate | 2000 |
| Orchardlea Foods Limited | Manufacturing | 1997 |
| Orcombe Ltd | Professional & scientific | 2015 |
| Orcombe Property Ltd | Real estate | 2015 |
| ORD Property Limited | Professional & scientific | 2025 |
| Orderlane Limited | Information & communication | 2026 |
| Ordonez Media Limited | Professional & scientific | 2021 |
| Orete Fast Ltd | Transport & logistics | 2025 |
| Organic Development Ltd | Information & communication | 2006 |