| Only Fools and Horses Ltd | Other services | 2016 |
| Only Fresh Ltd | Hospitality | 2026 |
| Only Hits Radio CIC | Information & communication | 2025 |
| Onlydans - Loaded Dog Limited | Hospitality | 2024 |
| Onlypay Ltd | Information & communication | 2023 |
| ONM Trading Ltd | Wholesale & retail | 2025 |
| Onofrei Limited | Transport & logistics | 2017 |
| ONP Associates Ltd | Professional & scientific | 2026 |
| Onpoint Homes Ltd | Real estate | 2024 |
| Onsite Recruitment Services Limited | Administrative & support | 2019 |
| Onslow Square (F3) Limited | Real estate | 2023 |
| Onspec Solutions Ltd | Administrative & support | 2023 |
| Onthego Eyecare Ltd | Health & social care | 2021 |
| Onthego Windscreens Ltd | Wholesale & retail | 2023 |
| Onuo Limited | Administrative & support | 2015 |
| Onya Global Ltd | Real estate | 2026 |
| Onyx Auto Care Ltd | Wholesale & retail | 2022 |
| Onyx Couriers Ltd | Transport & logistics | 2017 |
| Onyx Engagement Limited | Professional & scientific | 2026 |
| Onyx Integration Limited | Information & communication | 2010 |
| Onyx of Leicester Limited | Financial services | 2003 |
| Onyx Wholesale Ltd | Wholesale & retail | 2024 |
| Onyx Windows Limited | Construction | 2005 |
| Oobego Limited | Real estate | 2024 |
| Oodles Chinese 2 Limited | Hospitality | 2014 |
| Oodles Chinese Holdings Ltd | Wholesale & retail | 2016 |
| Oodles IP Limited | Professional & scientific | 2023 |
| Oomee Ltd | Wholesale & retail | 2024 |
| OON Clothing Ltd | Wholesale & retail | 2025 |
| OOO the Ltd | Financial services | 2025 |
| OP Capital Ltd | Manufacturing | 2019 |
| OP Living Ltd | Real estate | 2025 |
| Opaal Agency Ltd | Professional & scientific | 2008 |
| Opal Homes Limited | Construction | 2016 |
| Opal London Holdings Limited | Construction | 2021 |
| Opal Products Limited | Wholesale & retail | 2012 |
| Opal Software Solutions Limited | Information & communication | 2005 |
| OPAL22 Arts and ED C.I.C. | Administrative & support | 2016 |
| Opalite Properties Ltd | Real estate | 2026 |
| Oparaji Healthcare & Cleaning Services Ltd | Health & social care | 2016 |
| Opaxo Limited | Wholesale & retail | 2025 |
| Open Doorways Living Limited | Real estate | 2026 |
| Open Hands Trust (Leicester) | Education | 2006 |
| Open Mind Training & Development Limited | Professional & scientific | 2011 |
| Open Road Finance Ltd | Wholesale & retail | 2025 |
| Opengosim Limited | Information & communication | 2014 |
| Openroots CIC | Health & social care | 2025 |
| Operate Smart Consulting Ltd | Administrative & support | 2025 |
| Opes Tax Ltd | Professional & scientific | 2016 |
| Opexall Ltd | Professional & scientific | 2019 |
| Ophelia Aesthetics Ltd | Other services | 2020 |
| Ophelia Homes Limited | Construction | 2017 |
| Ophir Alliance Group Ltd | Financial services | 2024 |
| Opimus Ltd | Professional & scientific | 2011 |
| Opit Holdings Ltd | Administrative & support | 2019 |
| Opmen Limited | Information & communication | 2014 |
| Oporto Galo Ltd | Hospitality | 2017 |
| Oporto Sports Limited | Professional & scientific | 2012 |
| Opperman Plants Limited | Agriculture & fishing | 1990 |
| Oppia Limited | Information & communication | 2015 |
| Opronails Ltd | Wholesale & retail | 2026 |
| OPS VIP Transport Ltd | Transport & logistics | 2025 |
| Opti Mart Ltd | Wholesale & retail | 2023 |
| Optibay Limited | Other services | 2004 |
| Optic Utilities Ltd | Administrative & support | 2024 |
| OPTIC18 Ltd | Professional & scientific | 2014 |
| Optical 3 Limited | Wholesale & retail | 2007 |
| Optical Lounge Limited | Health & social care | 2019 |
| Optical People HQ Limited | Professional & scientific | 2025 |
| Opticare Solutions Limited | Professional & scientific | 2013 |
| Optics 2000 Limited | Health & social care | 1997 |
| Optima Advisory Ltd | Professional & scientific | 2013 |
| Optima Loughborough Limited | Extraterritorial | 2018 |
| Optima Perform Ltd | Health & social care | 2025 |
| Optimack Ltd | Construction | 2025 |
| Optimal Castings Ltd | Manufacturing | 2025 |
| Optimal PVT Ltd | Public administration | 2022 |
| Optimal Snagging Ltd | Construction | 2025 |
| Optimech Design Ltd | Professional & scientific | 2024 |
| Optimise Business Consulting Limited | Professional & scientific | 2017 |
| Optimised Transformations Ltd | Information & communication | 2023 |
| Optimist Real Estates Ltd | Real estate | 2026 |
| Optimisteeth Limited | Health & social care | 2024 |
| Optimize Lens Europe Limited | Manufacturing | 2026 |
| Optimize Lens UK Limited | Manufacturing | 2023 |
| Optimum Compliance Services Limited | Other services | 2017 |
| Optimum Path Financial Planning Limited | Professional & scientific | 1978 |
| Optimum Path Holdings Ltd | Professional & scientific | 1997 |
| Optimum Roofing Ltd | Construction | 2024 |
| Optimus Homeplus Ltd | Other services | 2024 |
| Optimus Living Limited | Real estate | 2022 |
| Optimus Motors Ltd | Wholesale & retail | 2009 |
| Optimus Properties Limited | Real estate | 2011 |
| Optimus Support Leicester Ltd | Other services | 2023 |
| Option Five Limited | Professional & scientific | 2014 |
| Option Suites Limited | Real estate | 2017 |
| Option Systems Limited | Information & communication | 1989 |
| Optionnet LLP | | 2010 |
| Optique Opticians Ltd | Wholesale & retail | 2002 |
| Optive Ltd. | Wholesale & retail | 2019 |