| Origin 8 Limited | Professional & scientific | 1996 |
| Origin Scaffolding Ltd | Construction | 2025 |
| Original Culture Events Ltd | Arts & recreation | 2021 |
| Orion Oilfield Services Limited | Professional & scientific | 2012 |
| Orion's Belt Design Limited | Manufacturing | 2024 |
| Orions Belt Holdings Limited | Financial services | 2020 |
| Orisma International Limited | Administrative & support | 2016 |
| Orlando Brand Ltd | Wholesale & retail | 2025 |
| Orlando Prime Ltd | Transport & logistics | 2023 |
| ORO Homes Ltd | Real estate | 2023 |
| Orpheon Leadership Limited | Professional & scientific | 2026 |
| ORS Bloodstock Limited | Arts & recreation | 2010 |
| ORS Investments Holdings Ltd | Financial services | 2023 |
| ORS Investments Ltd | Real estate | 2023 |
| ORS Ventures Holdings Ltd | Financial services | 2023 |
| ORS Ventures Ltd | Construction | 2023 |
| Orthopaedics and Trauma Surgery Expert Ltd | Health & social care | 2011 |
| Orthopaedics International Limited | Health & social care | 2017 |
| Orwell Village Stores Ltd | Extraterritorial | 2004 |
| Osasco Ltd | Health & social care | 2022 |
| Osborn Family Properties Limited | Real estate | 2001 |
| Oscan Ventures Ltd | Construction | 2016 |
| Osgr Limited | Professional & scientific | 2022 |
| Osprey Repairs Limited | Wholesale & retail | 2021 |
| Ostara Solutions Limited | Other services | 2014 |
| Oswald Sporting Limited | Wholesale & retail | 2017 |
| OT Guru Limited | Health & social care | 2017 |
| Othello Court Bedford Limited | Real estate | 1988 |
| Othello Investment Properties Ltd | Real estate | 2024 |
| Othello Solutions Limited | Wholesale & retail | 2006 |
| Othermind Digital Services Ltd | Information & communication | 2026 |
| Otis Physiotherapy Ltd | Health & social care | 2011 |
| Otis Solutions Ltd | Health & social care | 2024 |
| OTO Studio Limited | Wholesale & retail | 2026 |
| Otra Care Services Ltd | Health & social care | 2023 |
| OTS Ventures Ltd | Wholesale & retail | 2026 |
| Otter and Moose Limited | Wholesale & retail | 2017 |
| Oty.Agency Ltd | Professional & scientific | 2023 |
| Ouch Design Limited | Professional & scientific | 2006 |
| OUL Limited | Information & communication | 2014 |
| OUR Broker (UK) Ltd | Financial services | 2016 |
| OUR HR Ltd | Administrative & support | 2024 |
| OUR Lady Immaculate Catholic Academies Trust Ltd | Education | 2015 |
| OUR Next Step Limited | Financial services | 2021 |
| OUR Thsp Limited | Other services | 2021 |
| Ouse Holdings Ltd | Real estate | 2023 |
| Ouse Valley Agriculture Limited | Professional & scientific | 2023 |
| Ouse Valley Car Sales Ltd | Wholesale & retail | 2025 |
| Ouse Valley Estates Limited | Real estate | 2005 |
| Ouse Valley Property Ltd | Real estate | 2019 |
| OUT to Lunch Studio Limited | Information & communication | 2017 |
| Out-Growers Tanzania Ltd. | Hospitality | 2019 |
| Outfields Farm Ltd | Hospitality | 2011 |
| Outstay Ltd | Professional & scientific | 2026 |
| Outtabounds Media Limited | Information & communication | 2024 |
| Oversite Services Ltd | Construction | 2012 |
| Overslade Care Ltd | Health & social care | 2020 |
| Overtime Technologies Ltd | Information & communication | 2025 |
| Overton Consultancy Services Limited | Financial services | 2016 |
| Oviya Realty Limited | Real estate | 2022 |
| Owen Investments Ltd | Real estate | 2022 |
| Own Hand Services Ltd | Health & social care | 2016 |
| Own Physiotherapy Ltd | Health & social care | 2025 |
| Owner & Keeper Ltd | Information & communication | 2016 |
| Owusu-Ansah Limited | | 2015 |
| Oxdab Ltd | Information & communication | 2022 |
| Oxford Architectural Stone Limited | Wholesale & retail | 2020 |
| Oxford Health International Limited | Wholesale & retail | 2026 |
| Oxford Spires Management Co; Limited | Administrative & support | 1990 |
| Oxley Marston Limited | Professional & scientific | 2024 |
| Oxon Renewables Limited | Energy | 2014 |
| Oyster Renovations Limited | Construction | 2025 |
| O’Connor Scaffolding Services Ltd | Construction | 2021 |
| P & M Fabrications Limited | Manufacturing | 1988 |
| P & P Walker Limited | Construction | 2015 |
| P & P Walker Property Limited | Real estate | 2015 |
| P & R Domestic Limited | Construction | 2009 |
| P & T Care Support Services Ltd | Health & social care | 2018 |
| P a Simmonds Limited | Financial services | 2010 |
| P and R Trading Ltd | Wholesale & retail | 2009 |
| P B Fire Safety Limited | Professional & scientific | 2025 |
| P B Safety Consultancy Limited | Professional & scientific | 2003 |
| P Bains Surveying Ltd | Professional & scientific | 2023 |
| P C Physiotherapy Limited | Health & social care | 2014 |
| P Cornell Electrical & Building Services Ltd | Construction | 2012 |
| P Craddock & Associates Ltd | Professional & scientific | 2010 |
| P Daley Estates Ltd | Real estate | 2022 |
| P GUY Consulting Ltd | Construction | 2023 |
| P J Henry Holdings Limited | Professional & scientific | 2000 |
| P K Cleaning Services Limited | Administrative & support | 2009 |
| P K M Motors Limited | Wholesale & retail | 2023 |
| P of P Events Ltd | Hospitality | 2019 |
| P R Locks Limited | Other services | 2025 |
| P Rogers & SON Limited | Construction | 2025 |
| P Rzepkowski Ltd | Manufacturing | 2022 |
| P S Wright Electrical Limited | Construction | 2021 |
| P W Gates Distribution Limited | Transport & logistics | 2002 |
| P&R Group Holdings Limited | Financial services | 2022 |
| P&W Innovations Ltd | Information & communication | 2025 |
| P. C Crafts Limited | Manufacturing | 2013 |