| Oliver Songhurst Ltd | Construction | 2025 |
| Olli Ltd | Administrative & support | 2015 |
| OM Motor Company Limited | Wholesale & retail | 2024 |
| OM Private Ltd | Construction | 2022 |
| OM SAI Investments Limited | Financial services | 2023 |
| Ombra Drinks Limited | Extraterritorial | 2016 |
| Omdigitech Ltd | Administrative & support | 2024 |
| Omega Sail UK Limited | Administrative & support | 2009 |
| Omega Smiles Ltd | Health & social care | 2023 |
| Omenable Ltd | Real estate | 2022 |
| Omiyo Ltd | Wholesale & retail | 2025 |
| Omni Global Ltd | Administrative & support | 2022 |
| Omni Seasons Ltd | Construction | 2026 |
| Omnitask Global Holdings Ltd | Financial services | 2026 |
| Omnitask Global Solutions Ltd | Administrative & support | 2025 |
| Omnium International Legal Solutions Limited | Administrative & support | 2013 |
| Omorrj Company Limited | Health & social care | 2024 |
| ON Site Fuels Ltd | Wholesale & retail | 2013 |
| ON the Road Media Ltd | Information & communication | 2023 |
| ON the Spot Asset Limited | Real estate | 2023 |
| ONB Holding Ltd | Financial services | 2025 |
| One Hill Developments Limited | Real estate | 2020 |
| One I Productions Ltd | Arts & recreation | 2021 |
| One in Forty Ltd | Construction | 2019 |
| One Life Properties Limited | Construction | 2024 |
| One Linear Place Management Company Limited | Households | 2021 |
| One London Group Limited | Information & communication | 2026 |
| One SIX Eight Limited | Construction | 1997 |
| One SIX One Limited | Health & social care | 2001 |
| One Step (Support) Limited | Health & social care | 2002 |
| One Stop Plumbing Limited | Wholesale & retail | 2018 |
| One Vapes Ltd | Wholesale & retail | 2022 |
| One Weller Media Ltd. | Professional & scientific | 2017 |
| One Weller Properties Ltd | Real estate | 2026 |
| Oneness Ltd | Construction | 2024 |
| Onestring Ltd | Wholesale & retail | 2026 |
| Oneworld Productions Ltd | Information & communication | 2019 |
| Online Revu Ltd | Administrative & support | 2020 |
| Online UK Accountants Ltd | Professional & scientific | 2021 |
| Onlineluna Ltd | Other services | 2018 |
| Only Floors and Horses Ltd | Wholesale & retail | 2023 |
| Only One Scaffolding Ltd | Construction | 2026 |
| Ontheway Construction Limited | Real estate | 2022 |
| Ontime Build Limited | Construction | 2012 |
| Ontime Minicab Uxbridge Ltd | Transport & logistics | 2025 |
| Onyx Restaurant Ltd | Hospitality | 2025 |
| Open House West London Ltd | Real estate | 2023 |
| Open Network Exchange Ltd | Information & communication | 2021 |
| Openchat Labs Ltd | Information & communication | 2023 |
| Opera Cruise Limited | Transport & logistics | 2025 |
| Operant Ltd | Information & communication | 2022 |
| OPJ Ltd | Wholesale & retail | 2024 |
| OPM Incorporated Ltd | Real estate | 2025 |
| Optim Couriers Ltd | Transport & logistics | 2023 |
| Optimize Reach Limited | Professional & scientific | 2026 |
| Optimum Pallet North Ltd | Manufacturing | 2020 |
| Optimum Pallets Limited | Transport & logistics | 2016 |
| Optimum Recycling Limited | Water & waste | 2020 |
| Optipi Ltd | Information & communication | 2025 |
| Optix 360 Limited | Information & communication | 2026 |
| Opulent Abodes Limited | Hospitality | 2025 |
| Opupower Limited | Information & communication | 2021 |
| ORA Limited | Professional & scientific | 2017 |
| ORA Nest Ltd | Real estate | 2025 |
| Oracle Global Investments Ltd | Financial services | 2023 |
| Oracle Global Property Ltd | Real estate | 2025 |
| Orange Bear Productions Ltd | Information & communication | 2023 |
| Orange Clouds Limited | Real estate | 2021 |
| Orange Tree Property Limited | Real estate | 2026 |
| Oranjezicht Logistics Solutions Ltd | Transport & logistics | 2026 |
| Orbis Protect Limited | Administrative & support | 1990 |
| Orbit 24 Limited | Construction | 2024 |
| Orbit IOT Ltd | Wholesale & retail | 2024 |
| Orbit Trading Ltd | Wholesale & retail | 2025 |
| Orbital Basements Limited | Construction | 2014 |
| Orbital Properties Ltd | Real estate | 2016 |
| Orcalo Limited | Information & communication | 2019 |
| Orchard East Dulwich Grove Limited | Construction | 2016 |
| Orchard Homes Group Limited | Financial services | 2018 |
| Orchard Ilderton Road Ltd | Construction | 2017 |
| Orchard Leigh Villas Management Company Limited | Real estate | 1986 |
| Orchard Road Residents (Seer Green) Limited | Real estate | 1985 |
| Orchard SN Limited | Real estate | 2018 |
| Orchestra Cruise Limited | Transport & logistics | 2022 |
| Orcidea Ltd | Professional & scientific | 2025 |
| Ores Group Ltd | Other services | 2023 |
| Oriagen AI Ltd | Information & communication | 2025 |
| Oriental Carpet Brokers Limited | Professional & scientific | 1981 |
| Oriental Heritage Hall Limited | Hospitality | 2026 |
| Original Additions (Beauty Products) Limited | Wholesale & retail | 2004 |
| Original Additions Limited | Professional & scientific | 2007 |
| Original Additions Topco Limited | Professional & scientific | 2011 |
| Originalbrownboygamer Ltd | Real estate | 2025 |
| Orion F&B Limited | Wholesale & retail | 2021 |
| Orionbridge Ltd | Real estate | 2024 |
| Orlando Corporation Ltd | Transport & logistics | 2019 |
| Orme CBT Limited | Health & social care | 2015 |
| Ornamental Design Plastering Company Limited | Wholesale & retail | 1980 |
| Oromed International Ltd | Manufacturing | 2017 |
| Orsa Holdings Limited | Professional & scientific | 2002 |