| Optimal Aerodynamics Limited | Professional & scientific | 2001 |
| Optimal Cars Ltd | Wholesale & retail | 2018 |
| Optimal Performance Coaching Ltd | Education | 2025 |
| Optimal Performance Consultancy Ltd. | Health & social care | 2024 |
| Optimal Physical Therapy Limited | Health & social care | 2022 |
| Optimal Rentals & Renovations Ltd | Real estate | 2025 |
| Optimal Vision Recruitment Limited | Administrative & support | 2020 |
| Optimar Consulting Ltd | Professional & scientific | 2019 |
| Optimatech Limited | Professional & scientific | 2015 |
| Optimax Access Ltd | Wholesale & retail | 2017 |
| Optimise Your Future Ltd | Other services | 2020 |
| Optimiseall Limited | Information & communication | 2004 |
| Optimising Rehab Limited | Education | 2018 |
| Optimo Contracts Ltd | Administrative & support | 2025 |
| Optimum Abseiling Ltd | Construction | 2018 |
| Optimum Marine Limited | Manufacturing | 2014 |
| Optimum Pressure Washing Ltd | Other services | 2023 |
| Optimum Vehicle Management Ltd | Administrative & support | 2022 |
| Optimus Flowers Limited | Wholesale & retail | 2010 |
| Optinews Limited | Real estate | 1978 |
| Options Resourcing Bristol LLP | | 2011 |
| Options Resourcing Greater London LLP | | 2016 |
| Options Resourcing Ltd | Administrative & support | 2010 |
| Options Resourcing Solutions LLP | | 2026 |
| Options Resourcing Southampton LLP | | 2011 |
| Options Resourcing Sussex LLP | | 2011 |
| Optiprecision (Southampton) Limited | Professional & scientific | 1974 |
| Optirooms Service & Construction Ltd | Construction | 2025 |
| Optisoft Technologies Limited | Information & communication | 2025 |
| Optowireless Limited | Professional & scientific | 2024 |
| Opulence Financial Sourcing Ltd | Administrative & support | 2014 |
| Opulence Investment Management Limited | Professional & scientific | 2020 |
| Opus Psycheia Ltd | Administrative & support | 2017 |
| Opus X Properties Ltd | Real estate | 2024 |
| Opus X Property Ltd | Real estate | 2024 |
| OQI Maritime Outfitting Ltd | Manufacturing | 2010 |
| OR Debt Recovery Limited | Administrative & support | 2016 |
| OR-9 Access Ltd | Manufacturing | 2022 |
| Oracall Ltd | Wholesale & retail | 2011 |
| Orakzai Terah Limited | Wholesale & retail | 2025 |
| Orange Elephant Investments Limited | Financial services | 2017 |
| Orange Grove Hotel Ltd | Hospitality | 2010 |
| Orange Juice for Dinner Ltd | Other services | 2023 |
| Orange Rooms (Property) Limited | Real estate | 2002 |
| Orange Rooms (Southern) Limited | Hospitality | 2005 |
| Orange Rooms Group Limited | Financial services | 2014 |
| Orange Rooms Restaurants Southampton Limited | Administrative & support | 2004 |
| Orangepros Ltd | Construction | 2019 |
| Orba Investments Limited | Real estate | 1997 |
| Orbinet Ltd. | Information & communication | 2015 |
| Orbis Maritime Ltd | Transport & logistics | 2019 |
| Orbis Marketing Ltd | Administrative & support | 2010 |
| Orbit Flooring Ltd | Construction | 2009 |
| Orbit Workforce Solutions Ltd | Administrative & support | 2024 |
| Orbitco Limited | Hospitality | 2025 |
| Orbitol Ltd | Hospitality | 2025 |
| Orc's Kitchen Ltd | Manufacturing | 2024 |
| Orchard (Highwood Lane) Limited | Real estate | 2011 |
| Orchard (Special Projects) Limited | Construction | 2010 |
| Orchard Close (Botley) Limited | Extraterritorial | 2009 |
| Orchard Homes & Developments Limited | Construction | 1997 |
| Orchard Hotel Group Ltd | Hospitality | 2024 |
| Orchard House Investments Ltd | Real estate | 2020 |
| Orchard House Investments Operating Limited | Real estate | 2022 |
| Orchards Ground Care Ltd | Administrative & support | 2019 |
| Orchid Counselling Hub Limited | Health & social care | 2024 |
| Orchid Thai Spa Ltd | Health & social care | 2013 |
| Orchis Living Limited | Construction | 2015 |
| Ordinal Solution Ltd | Professional & scientific | 2025 |
| Ordnance Survey International Services Limited | Information & communication | 2016 |
| Ordnance Survey Leisure Limited | Wholesale & retail | 2009 |
| Ordnance Survey Limited | Professional & scientific | 2014 |
| Ordu Limited | Hospitality | 2016 |
| Orecruit Ltd | Administrative & support | 2025 |
| Orgature Limited | Wholesale & retail | 2024 |
| Orgcharthub Limited | Information & communication | 2018 |
| Orgill D Limited | Wholesale & retail | 2021 |
| Orhanco Limited | Hospitality | 2025 |
| ORI (UK) Ltd | Administrative & support | 2013 |
| Orian Hale Ltd | Wholesale & retail | 2026 |
| Oric Ltd | Information & communication | 2021 |
| Orient Express Productions Limited | Professional & scientific | 2005 |
| Oriental Merchant (Europe) Limited | Wholesale & retail | 2001 |
| Oriental Rugs of Bath Ltd | Wholesale & retail | 2009 |
| Origin Hair & Beauty Ltd | Other services | 2021 |
| Origin Intelligence Ltd | Information & communication | 2025 |
| Original Case Theatre Ltd | Administrative & support | 2020 |
| Originisproductions Ltd | Extraterritorial | 2021 |
| Orion Civil Engineering Solutions Limited | Construction | 2015 |
| Orion Delivery Ltd | Transport & logistics | 2018 |
| Orion Global Limited | Financial services | 2015 |
| Orion Safefacilities Ltd | Administrative & support | 2022 |
| Orion Safeguarding Limited | Administrative & support | 2017 |
| Orion Vehicle Solutions Ltd | Wholesale & retail | 2022 |
| Orionmonex Limited | Other services | 2020 |
| Orlik Property Limited | Real estate | 2021 |
| Orlov Properties and Developments Ltd | Construction | 2021 |
| Ormiston Steel Services Limited | Construction | 2016 |
| Ormiston-Adams Ltd | Manufacturing | 2021 |
| Ormitech Limited | Professional & scientific | 2020 |