| OPM Flexibles Limited | Manufacturing | 2005 |
| OPM Group Limited | Manufacturing | 2005 |
| Opnlytics Ltd | Information & communication | 2026 |
| Opodana Properties Limited | Real estate | 2025 |
| Oportunus Limited | Information & communication | 2014 |
| OPP Cleaning Ltd | Administrative & support | 2024 |
| Oppia Training Ltd | Professional & scientific | 2016 |
| Opran Transport Ltd | Transport & logistics | 2026 |
| OPS Exec Consulting Ltd | Professional & scientific | 2024 |
| Opscale Ltd | Administrative & support | 2025 |
| OPT Trading Limited | Administrative & support | 2014 |
| Opt-Ic Group Holdings Limited | Professional & scientific | 2025 |
| Opta Search Ltd | Administrative & support | 2024 |
| Optability Ltd | Professional & scientific | 2023 |
| Optagility Ltd | Information & communication | 2021 |
| Optalysys Ltd | Professional & scientific | 2013 |
| Opti-Innovations International Limited | Wholesale & retail | 2019 |
| Optic Labs Ltd | Information & communication | 2026 |
| Optic Networks Ltd | Information & communication | 2014 |
| Optica Security Systems Limited | Extraterritorial | 2012 |
| Opticaid UK Limited | Wholesale & retail | 1998 |
| Optical Connection Ltd | Health & social care | 2011 |
| Optical Propaganda Ltd | Wholesale & retail | 2022 |
| Optidentle Limited | Health & social care | 2025 |
| Optify Search Limited | Administrative & support | 2025 |
| Optik Trading Ltd | Wholesale & retail | 2021 |
| Optikaal Vision Care Centre Limited | Health & social care | 2013 |
| Optilima Ltd | Information & communication | 2025 |
| Optim Business Solutions Limited | Wholesale & retail | 2015 |
| Optima Defence & Security Group Limited | Public administration | 2009 |
| Optima Highways and Transportation Consultancy Limited | Administrative & support | 2010 |
| Optima Legal Services Limited | Professional & scientific | 2006 |
| Optima Leodis Limited | Construction | 2022 |
| Optimal Digital Services Ltd | Professional & scientific | 2022 |
| Optimal Healthcare Solutions Ltd | Health & social care | 2022 |
| Optimalik Coaching Ltd | Health & social care | 2022 |
| Optimas Properties Ltd | Real estate | 2016 |
| Optimisation Capital Ltd | Professional & scientific | 2025 |
| Optimise Your Property Limited | Professional & scientific | 2008 |
| OPTIMISTIC9JA Foods Ltd | Wholesale & retail | 2025 |
| Optimo Business Limited | Professional & scientific | 2019 |
| Optimum Foods Ltd | Hospitality | 2024 |
| Optimum Imaging Limited | Manufacturing | 2013 |
| Optimum Impressions Limited | Construction | 2003 |
| Optimum Medical Solutions Group Limited | Professional & scientific | 2013 |
| Optimum Medical Solutions Leeds Limited | Professional & scientific | 2013 |
| Optimum Medical Solutions Limited | Manufacturing | 2008 |
| Optimum Padel Ltd | Arts & recreation | 2025 |
| Optimum Performance Fitness Limited | Arts & recreation | 2014 |
| Optimum Recruit Limited | Administrative & support | 2009 |
| Optimum Recruitment Group Limited | Administrative & support | 2017 |
| OPTIMUM4 Limited | Professional & scientific | 1998 |
| Optimus CS Limited | Professional & scientific | 2015 |
| Optimus Studios Ltd | Manufacturing | 2019 |
| Optimus Vitrum Limited | Professional & scientific | 2018 |
| Optinet Solutions Limited | Administrative & support | 2024 |
| Option Box Limited | Administrative & support | 2011 |
| Optionbox Group Holdings Limited | Financial services | 2026 |
| Optionbox Networks Limited | Administrative & support | 2013 |
| Options (Rothwell) Limited | Manufacturing | 1990 |
| Options Hair Design Ltd | Other services | 2023 |
| Options of Options Limited | Financial services | 1994 |
| Optique Eyecare Limited | Health & social care | 2014 |
| Optirise UK Ltd | Information & communication | 2025 |
| Optiventra Limited | Information & communication | 2025 |
| Optivu Eyecare (Bramley) Ltd | Health & social care | 2017 |
| Optivu Eyecare (Morley) Ltd. | Health & social care | 2016 |
| Opttiuuq Limited | Wholesale & retail | 2011 |
| Opulence Haven Ltd | Wholesale & retail | 2025 |
| Opulence Leeds Limited | Other services | 2024 |
| Opulence Property Investments Ltd | Real estate | 2017 |
| Opulence Tiling Limited | Construction | 2024 |
| Opulent Delights Ltd | Wholesale & retail | 2024 |
| Opulux Life Ltd | Wholesale & retail | 2025 |
| Opus IP Limited | Information & communication | 2000 |
| Opus Smile Global Ltd | Administrative & support | 2025 |
| Opus Vinoteche Ltd | Hospitality | 2014 |
| ORA Films Limited | Professional & scientific | 2021 |
| ORA Films Subco Limited | Information & communication | 2023 |
| Oracle Concierge (London) Ltd. | Administrative & support | 2018 |
| Oracle Electrical Services Ltd | Construction | 2024 |
| Oracle Executive Services Limited | Administrative & support | 2022 |
| Oracle Mechanical & Electrical Limited | Construction | 2018 |
| Oracle Networks UK Ltd | Information & communication | 2008 |
| Oral Surgery Excellence Ltd | Health & social care | 2024 |
| Oranaise Leeds Ltd | Hospitality | 2019 |
| Orange Alliance Limited | Professional & scientific | 2023 |
| Orange Contract Cleaning Ltd | Administrative & support | 2011 |
| Orange Electrical Ltd | Manufacturing | 2020 |
| Orange Garden Strategy Ltd | Professional & scientific | 2024 |
| Orange Gym Limited | Arts & recreation | 2026 |
| Orange Properties (Leeds) Limited | Real estate | 2017 |
| Orange Smile Limited | Professional & scientific | 2007 |
| Orange Stripe Services Limited | Wholesale & retail | 2021 |
| Orange Surveying Services Ltd | Professional & scientific | 2022 |
| Oraytoons Ltd | Wholesale & retail | 2025 |
| Orbis Auto Care Ltd | Wholesale & retail | 2018 |
| Orbit Architectural Services Limited | Professional & scientific | 2021 |
| Orbit Company Group Limited | Health & social care | 2018 |
| Orbit Scaffolding Limited | Construction | 2008 |