| Oman Jewellers UK Ltd | Wholesale & retail | 2025 |
| OME Capital Ltd | Financial services | 2023 |
| Omega Hygiene Limited | Administrative & support | 2023 |
| Omni Entertainment Limited | Information & communication | 1993 |
| ON Maggie's Farm Ltd | Professional & scientific | 2021 |
| ON the Horizon Limited | Construction | 2007 |
| Onbaraka Ltd | Manufacturing | 2026 |
| One Day Investments Ltd | Real estate | 2021 |
| One Take Solutions Limited | Real estate | 2013 |
| Ongar Construction Limited | Construction | 2007 |
| Onium Consulting Ltd | Professional & scientific | 2024 |
| Open Automation Limited | Arts & recreation | 2024 |
| Open Limited | Professional & scientific | 2000 |
| Open Renewables Limited | Administrative & support | 2022 |
| Open Visual Data Ltd | Information & communication | 2023 |
| Openfourth Ltd | Information & communication | 2026 |
| Openport Limited | Real estate | 2006 |
| Operam Partners Ltd | Administrative & support | 2014 |
| Ophthalmic and Plastic Surgery London Ltd | Health & social care | 2023 |
| Optimac Europe Limited | Extraterritorial | 2012 |
| Optimal Landscapes Ltd | Administrative & support | 2023 |
| Orange Elephant Limited | Financial services | 2020 |
| Orange Things Ltd | Wholesale & retail | 2022 |
| Orange Walnut Limited | Wholesale & retail | 2011 |
| Oranger Ltd | Information & communication | 2015 |
| Orbit Brothers Ltd | Administrative & support | 2026 |
| Orbit Products Ltd | Wholesale & retail | 2022 |
| Orbital Security Limited | Administrative & support | 2016 |
| Orchard 5 Consulting Limited | Professional & scientific | 2015 |
| Orcion Systems Ltd | Information & communication | 2019 |
| Organised Chaos Limited | Arts & recreation | 2001 |
| Orienteer Mapazine Limited | Professional & scientific | 2022 |
| ORIGIN8 Design Limited | Arts & recreation | 2000 |
| Oriver & Co. UK Ltd | Administrative & support | 2014 |
| Orme Optics Ltd | Health & social care | 2023 |
| Ormo Ventures Ltd | Financial services | 2021 |
| Ormonde Abbey Investments Ltd | Administrative & support | 2026 |
| Orpine Consulting Limited | Professional & scientific | 2019 |
| Osbourne Villas Ltd | Real estate | 2026 |
| Osterley Park Property Limited | Construction | 2025 |
| Otism Ltd | Health & social care | 2026 |
| Outcomes Improved Ltd | Professional & scientific | 2019 |
| Outdoor Media Group Ltd | Professional & scientific | 2026 |
| Outlandish Music Services LLP | | 2009 |
| Outside Partner Ltd | Professional & scientific | 2016 |
| Outsider Research Limited | Professional & scientific | 2012 |
| OVI Global PVT Ltd | Wholesale & retail | 2024 |
| Owen Design Associates Limited | Professional & scientific | 1999 |
| Oxford Neurospine Limited | Health & social care | 2024 |
| Oxford School Online Ltd | Education | 2024 |
| OXY Group Ltd | Construction | 2025 |
| Oyster Property Holdings Limited | Real estate | 2021 |
| P & J G Leisure Ltd | Arts & recreation | 2026 |
| P & M Maintenance Limited | Construction | 2026 |
| P I Accountancy Bookeeping Services Limited | Professional & scientific | 2020 |
| P J Electrical & Home Automation Ltd | Construction | 2017 |
| P J Holmes Construction Limited | Construction | 2013 |
| P&D Media Ltd | Information & communication | 2017 |
| P&L Scale UP Properties Ltd | Real estate | 2023 |
| P.I. Accountancy Limited | Professional & scientific | 2011 |
| P1 Consulting Ltd | Professional & scientific | 2015 |
| P3R Engineers EOT Limited | Administrative & support | 2025 |
| PA Product Ventures Ltd | Information & communication | 2026 |
| Pabco Limited | Professional & scientific | 2021 |
| Padelio Ltd | Arts & recreation | 2025 |
| Paeds Pilates Physio Ltd | Health & social care | 2021 |
| Pageboy Films Ltd | Information & communication | 2011 |
| Palin Consultancy Ltd | Professional & scientific | 2015 |
| Palmer//Harding Ltd | Manufacturing | 2012 |
| Pangaea Ltd | Wholesale & retail | 2023 |
| Panoptes Intelligence Ltd | Information & communication | 2026 |
| Panoramic Developments Limited | Construction | 2015 |
| Panoramic Solutions Limited | Information & communication | 2014 |
| Panos PS Ltd | Other services | 2023 |
| Papillion Residential Limited | Real estate | 2020 |
| Papillon SAP Limited | Information & communication | 2018 |
| Para Grill Ltd | Hospitality | 2019 |
| Paramount Saas Finance Ltd | Professional & scientific | 2021 |
| Parent Therapy Hub Ltd | Health & social care | 2020 |
| Parisaei Women's Health Limited | Health & social care | 2015 |
| Park Road Manors Ltd | Construction | 2025 |
| Parks Property Ltd | Real estate | 2021 |
| Parmar HAM Co Limited | Professional & scientific | 2023 |
| Parmarbrook Urban Limited | Professional & scientific | 2016 |
| Parratt Properties Ltd | Real estate | 2020 |
| Parrish Attractions Ltd | Arts & recreation | 2025 |
| Parrish Leisure Limited | Arts & recreation | 2014 |
| Parsonage Farm Management Limited | Households | 1984 |
| Partydroid Merchandise Limited | Wholesale & retail | 2024 |
| PAS International Ltd | Information & communication | 2023 |
| Patgeh Properties Limited | Real estate | 2015 |
| Pathburner Ventures Limited | Information & communication | 2000 |
| Patronus Labs Ltd | Administrative & support | 2026 |
| Paul Esom Electrical Services Ltd | Construction | 2025 |
| Paul Moss Electrical Limited | Construction | 2007 |
| Paul Myers Project Management Limited | Construction | 2024 |
| Paya the Green Limited | Hospitality | 2025 |
| Paya Watford Limited | Hospitality | 2026 |
| Paytrader Ltd | Information & communication | 2026 |
| PBR Construction Ltd | Construction | 2019 |