| OPC Services Limited | Information & communication | 2025 |
| Opcode Consulting Ltd | Information & communication | 2024 |
| Open Circle Partners Ltd | Professional & scientific | 2019 |
| Open Source Contracting UK Ltd | Information & communication | 2019 |
| Openside Holdings Limited | Financial services | 2020 |
| Operam Managed Solutions Limited | Administrative & support | 2016 |
| Opex Construction Services Limited | Professional & scientific | 2013 |
| OPH Solutions Ltd | Information & communication | 2024 |
| Opple House Properties Ltd | Households | 2023 |
| Opsv Limited | Information & communication | 2017 |
| Optibelt (UK) Limited | Wholesale & retail | 1968 |
| Optical Resources Limited | Professional & scientific | 2008 |
| Opticore Communications Ltd | Administrative & support | 2017 |
| Optima Controls Limited | Administrative & support | 2010 |
| Optima DS Ltd | Construction | 2020 |
| Optimacode Technologies Limited | Information & communication | 2023 |
| Optimal Atlantic Ltd | Manufacturing | 2018 |
| Optimise Healthcare Franchise Limited | Health & social care | 2022 |
| Optimise Healthcare Group Ltd | Health & social care | 2018 |
| Optimise Risk Limited | Administrative & support | 2013 |
| Optimise Spinal Therapy Ltd | Health & social care | 2022 |
| Optimold Limited | Manufacturing | 2003 |
| Optimum Compliance Consultancy Limited | Information & communication | 2015 |
| Optimum Costs Consultants Limited | Professional & scientific | 2009 |
| Optimus Construction NW Limited | Construction | 2017 |
| Optimus Fitted Furniture Ltd | Construction | 2022 |
| Optionis Bidco Limited | Financial services | 2013 |
| Optionis Midco Limited | Financial services | 2013 |
| Optiva Digital Ltd | Professional & scientific | 2026 |
| Optonet Limited | Information & communication | 2015 |
| Opulence Salon Ltd | Other services | 2025 |
| Opulux Property Group Ltd | Real estate | 2023 |
| Opus Business Process Consulting Limited | Professional & scientific | 2014 |
| Opus Diem Consulting Limited | Professional & scientific | 2016 |
| Opus Engineering Solutions Ltd | Professional & scientific | 2026 |
| Opus Group (NW) Ltd | Financial services | 2022 |
| Opus Property Rentals Limited | Real estate | 2021 |
| Opus Recycling Solutions Ltd | Administrative & support | 2018 |
| Opus Traffic Management Solutions Ltd | Construction | 2020 |
| Opus Utility Solutions Ltd | Construction | 2002 |
| ORA Food Ltd | Hospitality | 2025 |
| Orange Bars Investments Ltd | Hospitality | 2024 |
| Orange Corner Interiors Ltd | Professional & scientific | 2025 |
| Orange Engineering Services Limited | Professional & scientific | 2019 |
| Orangebox 59 Ltd | Administrative & support | 2026 |
| Orbis Nova Group Ltd | Information & communication | 2024 |
| Orbit News Limited | Information & communication | 2006 |
| Orbit North Limited | Hospitality | 2026 |
| Orbit Park Ltd | Real estate | 2026 |
| Orbit Park NW Ltd | Real estate | 2026 |
| Orbital CAD Limited | Professional & scientific | 2016 |
| Orbital Networks Limited | Information & communication | 2008 |
| Orca Partnership Limited | Professional & scientific | 2014 |
| Orcha Health Ltd | Professional & scientific | 2014 |
| Orchard Consultancy Services Limited | Other services | 2018 |
| Orchard Dene Properties Limited | Real estate | 2023 |
| Orchard Grove Strategic Partners Ltd | Professional & scientific | 2019 |
| Orchard One Limited | Professional & scientific | 2014 |
| Orchard Technical Services Ltd | Professional & scientific | 2020 |
| Orchard Way Bramley Management Company Limited | Households | 2025 |
| Orchard Yard 2 Limited | Real estate | 2022 |
| Orchard Yard Limited | Real estate | 2020 |
| Orchid Financial Services Limited | Financial services | 2005 |
| Orde Properties Limited | Real estate | 2021 |
| Ordersoft Ltd | Information & communication | 2022 |
| Orex Competition NW Limited | Arts & recreation | 2019 |
| Orford 1964 Limited | Extraterritorial | 2013 |
| Orford Housing Limited | Real estate | 2017 |
| Orford Investments Limited | Construction | 1998 |
| Orford Lane Store Ltd | Wholesale & retail | 2025 |
| Orford Plastering and Building Limited | Construction | 2012 |
| Orford Technical Services Limited | Administrative & support | 2009 |
| Orgmented Ltd | Information & communication | 2025 |
| Orhanteh Limited | Wholesale & retail | 2025 |
| Orica Africa Holdings Limited | Administrative & support | 1998 |
| Orica UK Limited | Manufacturing | 1998 |
| Origin Healthcare Technologies Ltd | Information & communication | 2023 |
| Origin Medspa Ltd | Health & social care | 2026 |
| Original Cuts & Coffee Ltd | Hospitality | 2021 |
| Original Paul Kemp Ltd | Other services | 2025 |
| Original Race Equipment Limited | Manufacturing | 2022 |
| Orion Consulting (NW) Limited | Other services | 2018 |
| Orion Microbiology Limited | Professional & scientific | 2025 |
| Oris Dance Ltd | Arts & recreation | 2018 |
| Orka PAY Ltd | Information & communication | 2020 |
| Orke Properties Limited | Real estate | 2025 |
| Orkis Home Ltd | Wholesale & retail | 2025 |
| Orla Salon Ltd | Other services | 2024 |
| Orlando Team Ltd. | Construction | 2012 |
| Orlyo Ltd | Information & communication | 2026 |
| Orme Developments Ltd | Construction | 2016 |
| Ornela Marku Ltd | Real estate | 2024 |
| Orolea Ltd | Information & communication | 2016 |
| ORQUESTR8 Ltd | Professional & scientific | 2025 |
| Orrell Equine Veterinary Practice Limited | Professional & scientific | 2008 |
| Orrest Consulting Limited | Information & communication | 2016 |
| Orrson Recruitment Ltd | Professional & scientific | 2019 |
| Orthopaedic Endeavours Limited | Professional & scientific | 2017 |
| Orthrus Security Ltd | Administrative & support | 2025 |
| Orthrus Software Limited | Information & communication | 2021 |