| Oatlands Property Management Limited | Real estate | 2002 |
| Oatlands PUB Company Limited | Hospitality | 2017 |
| Oblia Group UK Ltd | Construction | 2017 |
| Octagon Construction Management Limited | Construction | 2025 |
| Octagon Development Management Limited | Construction | 2022 |
| Octagon Property Holdings Limited | Construction | 2024 |
| Octaplex Consultancy Ltd | Professional & scientific | 2012 |
| Odin Holdings (UK) Limited | Professional & scientific | 2016 |
| Odin-Spv Limited | Professional & scientific | 2024 |
| Odyssey Group Holdings Limited | Professional & scientific | 2003 |
| Odyssey Programming Limited | Education | 2024 |
| OFF2PADEL Limited | Arts & recreation | 2023 |
| Officeset Limited | Wholesale & retail | 1980 |
| Ogcs Global Ltd | Professional & scientific | 2012 |
| OH MEW Kids Limited | Wholesale & retail | 2021 |
| OHM Technology Global Ltd | Information & communication | 2025 |
| OHM Technology Services UK Ltd | Information & communication | 2025 |
| Old Burlington Limited | Administrative & support | 2025 |
| Old Farm Estates | Real estate | 1956 |
| Old Farm Properties | Real estate | 1962 |
| Old Rectory Secretarial Services Limited | Extraterritorial | 2000 |
| Old School Propco Ltd | Real estate | 2024 |
| Old School Trading Limited | Wholesale & retail | 2024 |
| Oldcot Limited | Real estate | 2015 |
| Oldfirm Limited | Professional & scientific | 2015 |
| Oligeo Property Lettings Limited | Real estate | 2024 |
| Oliphia Legal Services Limited | Professional & scientific | 2016 |
| Olive Property Developments Limited | Construction | 2014 |
| Olive Property Investments Limited | Real estate | 2018 |
| Omega International Associates | | 2003 |
| Omen Studio Ltd | Information & communication | 2025 |
| Ommg Network Limited | Other services | 2022 |
| Omnitech Consulting Limited | Professional & scientific | 2009 |
| Omsa Limited | Real estate | 2021 |
| ON Point Electrical Solutions Ltd | Construction | 2020 |
| Onalytica Limited | Information & communication | 2013 |
| Onboard Properties Limited | Households | 2021 |
| One Cobham Road Management Ltd | Construction | 2023 |
| One Global Recruitment Limited | Administrative & support | 2015 |
| One Hundred and Fifteen Ltd | Professional & scientific | 2015 |
| One Pilates Studios Limited | Arts & recreation | 2023 |
| Onepad Ltd | Construction | 2012 |
| Ongar Close Limited | Other services | 1997 |
| Online Courses by Mako Ltd | Education | 2021 |
| Online Switch Ltd | Energy | 2025 |
| Only Rods and Hoses Ltd. | Water & waste | 2013 |
| Opal & Teak Hair Extensions Ltd | Other services | 2024 |
| Open a Book Ltd | Information & communication | 2020 |
| Opera AI Limited | Professional & scientific | 2025 |
| OPS Excellence Ltd | Professional & scientific | 2007 |
| Optic Media Ltd | Information & communication | 2019 |
| Optimise Physiotherapy Limited | Health & social care | 2015 |
| Optimised Physio Solutions Ltd | Health & social care | 2022 |
| Optisoul Ltd | Construction | 2015 |
| Opto Impact Ventures Ltd | Financial services | 2020 |
| Opus Meridian Ltd | Professional & scientific | 2026 |
| Oral Creations Dental Laboratory Ltd | Manufacturing | 2009 |
| Oral Marketing Ltd | Professional & scientific | 2024 |
| Orang Grove Care Limited | Health & social care | 2023 |
| Orangery Care Ltd | Health & social care | 2010 |
| Orb-Pay Ltd | Other services | 2021 |
| Orbit Works Limited | Information & communication | 2025 |
| Orchard Close (Walton on Thames) Management Company Limited | Households | 2008 |
| Orchard House Loxwood Limited | Construction | 2025 |
| Order Square Ltd | Information & communication | 2023 |
| Oreal Ltd | Other services | 2024 |
| Organic and Natural Ltd | Extraterritorial | 2014 |
| Oriental Express Yang Ltd | Hospitality | 2016 |
| Orion Training (International) Limited | Professional & scientific | 2020 |
| Orkli (UK) Limited | Wholesale & retail | 1996 |
| Ormiston Consulting Limited | Information & communication | 2024 |
| Ormonde Gate Garden Limited | Households | 2004 |
| Orthodocs Consultancy Limited | Health & social care | 2000 |
| Osato Distribution World Limited | Manufacturing | 2012 |
| Osborne and Sons Building Services Ltd | Construction | 2022 |
| Osterley Skin Care Limited | Other services | 2013 |
| Otis Inc Limited | Arts & recreation | 2018 |
| OTS Future Ltd | Real estate | 2024 |
| OUT of the Woods Consultancy Ltd | Professional & scientific | 2019 |
| Outdoor Media Limited | Information & communication | 2003 |
| Outeniqua Advisory Limited | Professional & scientific | 2026 |
| Outlook Press Limited | Professional & scientific | 1989 |
| Outram Place Management Limited | Households | 2025 |
| OVE Care Limited | Information & communication | 2023 |
| Oveli Healthcare Limited | Health & social care | 2018 |
| Overissue Limited | Professional & scientific | 1998 |
| Overmist Developments Limited | Construction | 2016 |
| OYM Creatives Ltd | Information & communication | 2024 |
| P I (UK) Limited | Manufacturing | 2001 |
| P J Norris Limited | Construction | 2019 |
| P&N Leasing Ltd | Administrative & support | 2023 |
| P-Gee Music Limited | Arts & recreation | 1995 |
| P.A.N. Holdings Limited | Professional & scientific | 1985 |
| P.S & S.K Lohia Investments Ltd | Professional & scientific | 2019 |
| P10 Accountancy Ltd | Professional & scientific | 2021 |
| P10 Financial Ltd | Financial services | 2015 |
| P11 Associates Limited | Administrative & support | 2020 |
| P3 Consultants Limited | Other services | 2017 |
| PA IT Solutions Limited | Administrative & support | 2014 |
| Pace 70 Limited | Professional & scientific | 2021 |