| Open Architecture (London) Ltd | Professional & scientific | 2012 |
| Open Tech IT Ltd | Information & communication | 2021 |
| Opopa Ltd | Construction | 2024 |
| Optidev Solutions Limited | Information & communication | 2016 |
| Optimal Care Concept Limited | Administrative & support | 2016 |
| Optimal Wellness Services Ltd | Wholesale & retail | 2021 |
| Optimed Consol Ltd | Wholesale & retail | 2022 |
| Optimise Building Consultancy Limited | Professional & scientific | 2009 |
| Optymise Ltd | Real estate | 2019 |
| Opus Facilities Limited | Extraterritorial | 2018 |
| Opus Waterproofing Solutions Limited | Construction | 2004 |
| Oqqo Limited | Professional & scientific | 2022 |
| Oracle Automotive Ltd | Wholesale & retail | 2019 |
| Oracleay Limited | Health & social care | 2023 |
| Orakings Limited | Professional & scientific | 2012 |
| Orakings Properties Limited | Real estate | 2021 |
| Orange Coffee Shop Limited | Hospitality | 2024 |
| Orange Express Ltd | Transport & logistics | 2011 |
| Orange Records Limited | Arts & recreation | 1969 |
| Orange Songs Limited | Arts & recreation | 1977 |
| Oraword Publishers Limited | Information & communication | 2003 |
| Orbis Financial Limited | Financial services | 2021 |
| Orca App Ltd | Information & communication | 2019 |
| Orchard Autos Limited | Wholesale & retail | 2001 |
| Orchard Court Management Company Limited | Households | 1984 |
| Orchard Garage Limited | Wholesale & retail | 2002 |
| Orchard Ingredients Limited | Professional & scientific | 2010 |
| Orchard Park (1989) Management Co. Limited | Households | 1989 |
| Orchard Park Management Co. Limited | Real estate | 1987 |
| Orchard Plumbing and Heating Limited | Professional & scientific | 2003 |
| Orchard Plumbing Limited | Construction | 1999 |
| Orchard Secure Storage Limited | Administrative & support | 2019 |
| Orchard View Management Company (Kent) Limited | Households | 2024 |
| Orchestra of the City Productions Limited | Arts & recreation | 2025 |
| Orchid Ventures Limited | Real estate | 2021 |
| Ordon Ltd | Construction | 2018 |
| Oreed Group Ltd | Professional & scientific | 2008 |
| Orginate Ltd | Information & communication | 2022 |
| Oriental Daliy Fresh Store Ltd | Wholesale & retail | 2022 |
| Origin Electrical Limited | Professional & scientific | 2019 |
| Original Portraits Limited | Wholesale & retail | 2025 |
| Orinlak Limited | Real estate | 2023 |
| Oriowo Consultancy Ltd | Professional & scientific | 2023 |
| Oritoke Skylogistics Ltd | Transport & logistics | 2025 |
| Orobosa Group International Limited | Real estate | 2017 |
| Orobosa Solicitors Limited | Professional & scientific | 2021 |
| Orpington Catering Co Ltd | Hospitality | 2023 |
| Orren Group Ltd | Wholesale & retail | 2025 |
| Orthosmile Clinic Limited | Extraterritorial | 2019 |
| Orthosmile Earls Court Ltd | Health & social care | 2018 |
| Orthosmile Hounslow Limited | Health & social care | 2018 |
| Orthosmile Limited | Extraterritorial | 1999 |
| OS Consulting Ltd | Professional & scientific | 2026 |
| Osac Holdings Ltd | Financial services | 2025 |
| Oscar Prop Ltd | Real estate | 2020 |
| OSE & VAL Transport and Logistics Ltd | Transport & logistics | 2025 |
| OSE Logistics Ltd | Transport & logistics | 2026 |
| Osebo Limited | Wholesale & retail | 2017 |
| Oseiga Limited | Information & communication | 2018 |
| Osho Anandbodhi Meditation Centre | Other services | 2018 |
| Osita Nwegbu Photography Limited | Professional & scientific | 2013 |
| Oska Construction Services Ltd | Construction | 2012 |
| OSN Limited | Information & communication | 2014 |
| Ostrie Ltd | Transport & logistics | 2017 |
| Osvic Ltd | Construction | 2021 |
| OT Publishing Limited | Information & communication | 2003 |
| Otherside Films Ltd | Information & communication | 2022 |
| Othm Ltd | Extraterritorial | 2026 |
| Othm Qualifications | Education | 2003 |
| Otters Pocket Limited | Real estate | 2026 |
| OUD Khaleeji Limited | Wholesale & retail | 2025 |
| OUR Bar, Your Place Ltd | Hospitality | 2020 |
| OUR Focus Roofing Ltd | Construction | 2025 |
| OUR Held Space CIC | Education | 2025 |
| OUR Lean Journey Limited | Information & communication | 2023 |
| Ourstylez Limited | Wholesale & retail | 2017 |
| Out-It Limited | Information & communication | 2025 |
| Outdoor Living (Partners) Ltd | Construction | 2026 |
| Outdoorcovers Ltd | Wholesale & retail | 2017 |
| Outer-Intuition Limited | Other services | 2013 |
| Output Electrical Ltd | Construction | 2018 |
| Oven DOC Ltd | Administrative & support | 2024 |
| Overall Company Limited | Real estate | 2024 |
| Owas Accountants Limited | Professional & scientific | 2026 |
| Owls Motor Club Limited | Arts & recreation | 1984 |
| Own Your Wings Ltd | Administrative & support | 2024 |
| OX Enterprise Management Ltd | Hospitality | 2026 |
| Oxen Capital Consultants Ltd | Professional & scientific | 2018 |
| Oxford Tech Investment Ltd | Financial services | 2024 |
| Oxleas Management Services Ltd | Administrative & support | 2026 |
| Oxleas Prison Services Limited | Health & social care | 2015 |
| Oxleas Tree Care Ltd | Administrative & support | 2012 |
| Oyejobi Handyman Ltd | Transport & logistics | 2021 |
| Oyeronke Fowowe Limited | Information & communication | 2022 |
| Ozone Lift Consultancy Limited | Professional & scientific | 2022 |
| P & a Management Services Ltd | Construction | 2015 |
| P & T Communications Limited | Information & communication | 2018 |
| P & T Fitness Ltd | Other services | 2025 |
| P & W Johnson Limited | Wholesale & retail | 2003 |
| P K News&Convenience Store Ltd | Wholesale & retail | 2018 |