| ON Track Learning Limited | Administrative & support | 2024 |
| Onbrand Collective Limited | Professional & scientific | 2026 |
| One Brightly Cyber Ltd | Wholesale & retail | 2018 |
| One Earth Renewables Limited | Administrative & support | 2012 |
| One TEN Events Limited | Administrative & support | 2018 |
| Oneness Wellness Ltd | Public administration | 2022 |
| Oneplanet Solutions Limited | Professional & scientific | 2010 |
| Online Photo Services Ltd | Manufacturing | 2017 |
| Online Sports Lounge Ltd | Education | 2024 |
| Only Hope Project Ltd | Information & communication | 2021 |
| Only Paws Hydro Fitness Limited | Professional & scientific | 2025 |
| Only Research Group Limited | Wholesale & retail | 2026 |
| Onslow & Crisp Ltd | Manufacturing | 2024 |
| Ontrack Groundwork Services Ltd | Construction | 2024 |
| Open (Creative) Ltd | Construction | 2021 |
| Open Design Studio Ltd | Professional & scientific | 2011 |
| Opening Move Ltd. | Administrative & support | 2009 |
| Opera Brava Limited | Arts & recreation | 1989 |
| Operation Forward Assist Ltd | Health & social care | 2024 |
| Operation Wolf Ltd | Wholesale & retail | 2022 |
| Operation Wolf Ventures Ltd | Professional & scientific | 2023 |
| Opex Consultants (UK) Limited | Professional & scientific | 2014 |
| Ophelia Art Consultancy Limited | Arts & recreation | 2022 |
| Optimum Laser Hair Removal Ltd | Other services | 2021 |
| Optimum People Ltd | Administrative & support | 2025 |
| Optopia Ltd | Professional & scientific | 2021 |
| Opus Electrical Limited | Construction | 2020 |
| Oracle - Practice Business Solutions Ltd. | Education | 2008 |
| Orange Court Limited | Wholesale & retail | 1995 |
| Orbinox (UK) Limited | Wholesale & retail | 2003 |
| Orchard Barn Management Company Limited | Households | 2019 |
| Orchard Gate Ltd | Real estate | 2014 |
| Orchard Rise Investments Limited | Real estate | 2020 |
| Orchard Way Ltd | Real estate | 2007 |
| Ordo Space Limited | Professional & scientific | 2022 |
| Orestone Limited | Real estate | 2024 |
| Orestone Properties LLP | | 2020 |
| Organic Gardener Ltd | Wholesale & retail | 2025 |
| Organic Universe Limited | Administrative & support | 1998 |
| Oriental Tea Merchant Ltd | Manufacturing | 2023 |
| Origin Case Ltd | Information & communication | 2018 |
| Orion Developments (Southern) Ltd | Real estate | 2004 |
| Orion Holdings (Chichester) Limited | Extraterritorial | 1999 |
| Orion MIS Ltd | Administrative & support | 2018 |
| Orkadian Wavemaster Ltd | Information & communication | 2020 |
| Orpington Timber & Building Supplies Limited | Extraterritorial | 1988 |
| Osborne Business Enterprises Ltd | Manufacturing | 2003 |
| Oshea Removals Ltd | Transport & logistics | 2025 |
| Osoliving Limited | Wholesale & retail | 2015 |
| Osowarm Limited | Extraterritorial | 2011 |
| Osteoesprit Ltd | Health & social care | 2017 |
| Otto Cases Ltd | Wholesale & retail | 2026 |
| OUR Financial REP Limited | Professional & scientific | 2013 |
| OUR Right to Read | Education | 2003 |
| Outback Jack Limited | Real estate | 2006 |
| Outdoordesign Limited | Other services | 2004 |
| Outstanding Valet & Detail Ltd | Wholesale & retail | 2023 |
| Over the Top Golf Ltd | Wholesale & retail | 2018 |
| Overland Competitions Ltd | Wholesale & retail | 2024 |
| Overordinary Ltd | Manufacturing | 2019 |
| Overwatch Energy & Technology Limited | Professional & scientific | 2024 |
| Oving Properties Limited | Real estate | 2016 |
| Owen - Powell Ltd | Professional & scientific | 2008 |
| Owen Advertising Limited | Professional & scientific | 1976 |
| Owen B2B2C Limited | Other services | 2017 |
| Owen Communications Limited | Other services | 1985 |
| Owen Conservatories Ltd | Construction | 2022 |
| Owen Group International Limited | Financial services | 2017 |
| Oxford Car Detailing Ltd | Wholesale & retail | 2025 |
| Oxford Doorsets Ltd | Construction | 2023 |
| Oxford Global Construction Recruitment Limited | Administrative & support | 2008 |
| Oxmarket Centre of Arts Limited | Arts & recreation | 1971 |
| Oxmarket Trading Limited | Wholesale & retail | 2021 |
| Oxygen Designs Limited | Professional & scientific | 2016 |
| Oyster Logistics & Removals Limited | Transport & logistics | 2016 |
| OZ Aladdin Limited | Wholesale & retail | 2017 |
| P & H Antill Limited | Real estate | 2024 |
| P Flowers Carpentry & Joinery Ltd | Construction | 2020 |
| P J LEE Roofing Ltd | Construction | 2015 |
| P J Yeates Property Ltd | Construction | 2020 |
| P of W Group Limited | Real estate | 2016 |
| P&X Restaurants Ltd | Hospitality | 2020 |
| P. and R. Enterprises (Hampshire) Limited | Real estate | 1968 |
| P. C. Petter & SON Limited | Financial services | 2020 |
| P. C. Sports Science Services Ltd | Administrative & support | 2011 |
| P. Phillips Contractors Limited | Construction | 2006 |
| P.A. Fire Systems Limited | Public administration | 2012 |
| P.A. Hayers Limited | Other services | 2002 |
| P.L. Estates ANU Ltd | Real estate | 2011 |
| P4 Skala Limited | Real estate | 2015 |
| PA ARK Holdings Limited | Financial services | 2015 |
| PA ARK Projects Limited | Health & social care | 2012 |
| Pablo.Pl Ltd | Transport & logistics | 2023 |
| Pablon Ltd | Manufacturing | 2018 |
| Pacaroca Ltd | Information & communication | 2024 |
| Pachotrans Ltd | Transport & logistics | 2025 |
| Pacific Decision Sciences Europe Limited | Information & communication | 1995 |
| Pack Horse PUB Ltd | Hospitality | 2024 |
| Paddock Prestige Ltd | Administrative & support | 2023 |
| Paddys Plaice Ltd | Hospitality | 2022 |