| Orso Medical Ltd | Health & social care | 2019 |
| Orta Solar Farms Limited | Professional & scientific | 2011 |
| Ortho 2008 Limited | Health & social care | 2006 |
| Orthopaedic Trauma Solutions Ltd | Health & social care | 2019 |
| Orthoscene Limited | Health & social care | 2006 |
| Orthvil Limited | Real estate | 2024 |
| Ortino Limited | Information & communication | 2007 |
| Orton Financial Ltd | Financial services | 2022 |
| Ortona Assets Ltd | Wholesale & retail | 2021 |
| Ortus PSR Ltd | Administrative & support | 2020 |
| Orwell Design Associates Limited | Professional & scientific | 2015 |
| Orwell Grange Ltd | Real estate | 2022 |
| Orwell Vets Limited | Extraterritorial | 2003 |
| Oryx (Executive Search) Limited | Administrative & support | 1991 |
| Oryx Scaffold Design Ltd | Administrative & support | 2017 |
| ORYX43 Ltd | Information & communication | 2016 |
| Orzac Services Ltd | Transport & logistics | 2019 |
| OS Property Services Limited | Real estate | 2015 |
| OSA Healthcare Recruitment Ltd | Administrative & support | 2019 |
| Osaco UK Ltd | Professional & scientific | 2023 |
| Osadare Limited | Construction | 2018 |
| Osapire Enterprise Limited | Administrative & support | 2025 |
| Osborne Bricklaying Contractors Ltd | Construction | 2020 |
| Osborne Care Community Ltd | Education | 2018 |
| Osborne Care Holdings Ltd | Health & social care | 2019 |
| Osborne Care Homes Limited | Health & social care | 1989 |
| Osborne Clarke International Services Limited | Administrative & support | 2012 |
| Osborne Clarke Legal Services Limited | Professional & scientific | 1986 |
| Osborne Clarke Nominees Limited | Professional & scientific | 1994 |
| Osborne Clarke Trustees Limited | Professional & scientific | 1995 |
| Osborne Consulting Limited | Professional & scientific | 2026 |
| Osborne LU Product Services Ltd. | Wholesale & retail | 2012 |
| Osborne Mansions Property Management Company Limited | Professional & scientific | 1997 |
| Osborne Property (Glos) Limited | Real estate | 2016 |
| Osborne Rail Services Ltd | Transport & logistics | 2018 |
| Osborne Restoration Limited | Manufacturing | 2019 |
| Osborne Wallis House Management Limited | Households | 1998 |
| Oscar & Osterly Services Limited | Transport & logistics | 2024 |
| Oscar's Driving School Limited | Other services | 2017 |
| Oscar's Talent Agency Limited | Administrative & support | 2025 |
| Oscaroos LLP | | 2016 |
| Oscarsam Limited | Real estate | 2021 |
| Oscatel Ltd | Information & communication | 2009 |
| Osenso Ltd | Wholesale & retail | 2024 |
| Osgard Homes Limited | Real estate | 2020 |
| Oshares Tech Limited | Information & communication | 2026 |
| Oshi Design Ltd | Professional & scientific | 2022 |
| Osiris Rail Ltd | Administrative & support | 2022 |
| OSJ Consultants Ltd | Professional & scientific | 2024 |
| Osmaston Road Dental Care Ltd | Health & social care | 2011 |
| Osmond Tricks Limited | Real estate | 2018 |
| Osmond Weedon Limited | Construction | 2021 |
| OSO Media Ltd | Information & communication | 2018 |
| Osphil Ltd | Other services | 2024 |
| Osprey Business Systems Limited | Extraterritorial | 1996 |
| Osprey Court Ltd | Construction | 2020 |
| Osprey Developments Limited | Wholesale & retail | 1970 |
| Osprey Heavy Lift Limited | Construction | 2015 |
| Osprey Maritime Limited | Transport & logistics | 1995 |
| Osprey Park (Flats 23 to 28) Management Company Limited | Real estate | 2007 |
| Osprey Shipping Limited | Transport & logistics | 1979 |
| Osric Finance Limited | Real estate | 1962 |
| Ossenge Limited | Health & social care | 2019 |
| Ossie Consulting Ltd | Extraterritorial | 2024 |
| Ossilly Properties Limited | Real estate | 2014 |
| Ostara Technologies Limited | Manufacturing | 2024 |
| Osteon Lutra Ltd | Financial services | 2017 |
| Oster Property Ltd | Real estate | 2022 |
| Ostle & Wilson Ltd | Hospitality | 2016 |
| Ostratto Ltd | Information & communication | 2020 |
| OSW Investments Galloper Jvco (UK) Limited | Financial services | 2022 |
| OSW Investments Korean Jvco 1 Limited | Financial services | 2022 |
| OSW Investments Korean Jvco 2 Limited | Financial services | 2022 |
| OSW Investments Korean Jvco 3 Limited | Financial services | 2022 |
| OSW Investments Korean Jvco 4 Limited | Financial services | 2022 |
| OSW Investments Korean Jvco 5 Limited | Financial services | 2022 |
| OSW Investments Rampion Jvco (UK) Limited | Financial services | 2022 |
| Oswestry Dental Laboratory Limited | Manufacturing | 2004 |
| Oswin Davis FM Ltd | Construction | 2022 |
| Osylat Trading Limited | Other services | 2024 |
| OT Fitness Limited | Health & social care | 2013 |
| Ot-Vation Ltd | Extraterritorial | 2022 |
| OTA Engineering Ltd | Professional & scientific | 2026 |
| Otaku Development Limited | Information & communication | 2016 |
| Otaria Consulting Ltd | Administrative & support | 2016 |
| OTF Services Ltd | Administrative & support | 2024 |
| OTH Motors Ltd | Wholesale & retail | 2021 |
| Other Fish Ltd | Wholesale & retail | 2019 |
| Other Restaurant Bristol Ltd | Hospitality | 2023 |
| Otherways Limited | Arts & recreation | 2007 |
| Otherworld Tattoos Ltd | Arts & recreation | 2025 |
| Otion Ltd. | Extraterritorial | 2021 |
| Otley Solutions Ltd | Professional & scientific | 2020 |
| OTM Construction Limited | Construction | 2023 |
| Otoo Foundation CIC | Education | 2025 |
| Otoutdoors CIC | Health & social care | 2022 |
| OTR Transport Ltd | Transport & logistics | 2011 |
| Otrovez Limited | Professional & scientific | 2012 |
| OTS Digital Ltd | Wholesale & retail | 2022 |
| Otso Retail Design Limited | Professional & scientific | 2024 |