| Old Hall Cressage Limited | Hospitality | 2019 |
| Old House Holdings Limited | Financial services | 2024 |
| Old Mill Close (Worthen) Management Company Limited | Households | 2016 |
| Old Plough Management Company Limited | Real estate | 2024 |
| Old Telephone Exchange (Newton Abbot) Limited | Households | 2012 |
| Olitory Limited | Manufacturing | 2021 |
| Olive & Rose Interiors Ltd | Professional & scientific | 2023 |
| Olive Joyce Consulting Limited | Professional & scientific | 2017 |
| Olive Joyce Estates Limited | Real estate | 2022 |
| Olivers Point Rentals Limited | Real estate | 2025 |
| Olivia Jade Sewing Ltd | Wholesale & retail | 2026 |
| Olivia White Consulting Limited | Professional & scientific | 2019 |
| Omari Tech Limited | Wholesale & retail | 2018 |
| Omega - Ending Isolation, Ending Loneliness Ltd | Health & social care | 2007 |
| Omega VFX Ltd | Information & communication | 2021 |
| Omni Services Limited | Administrative & support | 2025 |
| ON Consultancy Ltd. | Professional & scientific | 2002 |
| ON Demand Healthcare Solutions Limited | Administrative & support | 2017 |
| ON Point Driving Solutions Ltd | Transport & logistics | 2025 |
| ON the DOT Carz Ltd | Transport & logistics | 2025 |
| ON Time Hydraulics Ltd | Manufacturing | 2018 |
| On-Demand Healthcare Infrastructure Solutions Limited | Administrative & support | 2017 |
| Once Again Ltd | Wholesale & retail | 2017 |
| Oncology Systems Limited | Wholesale & retail | 1999 |
| One and ALL Design Ltd | Professional & scientific | 2025 |
| One Fifty Properties Limited | Real estate | 2021 |
| One Fish Street Limited | Households | 2018 |
| One for Sorrow Tattoo Ltd | Other services | 2019 |
| One GAS Medic Limited | Health & social care | 2015 |
| One Golf League Ltd | Arts & recreation | 2023 |
| One in a Million Designs Limited | Professional & scientific | 2008 |
| One to Learn CIC | Education | 2012 |
| Onehomes Property Ltd | Real estate | 2020 |
| Onetwentyone East Residents Company Limited | Households | 2003 |
| Online Home Improvements Ltd | Wholesale & retail | 2019 |
| Online Transport Archive | Arts & recreation | 2000 |
| Onny Cottage Cabins Limited | Hospitality | 2021 |
| Opdg Limited | Manufacturing | 2024 |
| Open Harmony CIC | Education | 2019 |
| Open Space (Anstey) Management Limited | Real estate | 2017 |
| Open Space (Ward Street) Management Limited | Real estate | 2017 |
| Open Space Management Limited | Real estate | 2015 |
| Operalis Limited | Financial services | 2024 |
| Operations Assessment Group Ltd. | Professional & scientific | 2017 |
| OPH Construction Ltd | Manufacturing | 2016 |
| Optima Facilities Management Ltd | Administrative & support | 2024 |
| Optimal Movement Ltd | Health & social care | 2017 |
| Optr Ltd | Information & communication | 2025 |
| Opulence Living Solutions Ltd | Construction | 2022 |
| Opus Power Limited | Construction | 2023 |
| Opus Vitae Consultancy Limited | Administrative & support | 2018 |
| Oracle Dental Limited | Health & social care | 2014 |
| Oracle Shrewsbury Ltd | Health & social care | 2026 |
| Oralall Care Ltd | Health & social care | 2019 |
| Orama Management Solutions Limited | Professional & scientific | 2016 |
| Orange Seal Limited | Construction | 2021 |
| Orangeleaf Systems Limited | Information & communication | 1998 |
| Orbicom Group Limited | Education | 1990 |
| Orca Five Limited | Financial services | 2023 |
| Orca Nine Limited | Financial services | 2022 |
| Orchard Early Years Ltd | Health & social care | 2014 |
| Orchard Fields (Shavington) Management Company Limited | Real estate | 2017 |
| Orchard Retreat SY5 Ltd | Hospitality | 2024 |
| Orchard Site Solutions Ltd | Construction | 2026 |
| Orchardleaze (Stroud) Management Company Limited | Households | 2008 |
| Orchidsandbeyond Ltd | Agriculture & fishing | 2024 |
| Orchidvale (UK) Limited | Construction | 2019 |
| Ordos Holding Ltd | Wholesale & retail | 2024 |
| Organic Energy (UK) Limited | Wholesale & retail | 2006 |
| Organic Farmers and Growers C.I.C. | Other services | 1975 |
| Organic Resource Agency Ltd. | Agriculture & fishing | 1996 |
| Orgreen Optics UK Limited | Manufacturing | 2021 |
| Oriental Food Express Limited | Manufacturing | 2011 |
| Oriental Little Camel Ltd. | Wholesale & retail | 2022 |
| Original Ltd | Manufacturing | 2003 |
| ORP Surveillance Ltd | Administrative & support | 2002 |
| Orthentic Smiles Limited | Health & social care | 2018 |
| Orthographics Ltd | Information & communication | 2023 |
| Ortra IT Consultancy Ltd | Information & communication | 2022 |
| Osborne Close Management Company Limited | Households | 2008 |
| Osborne Quarters (Management Company) Ltd | Households | 2019 |
| Oscar's Chinese Takeaway Limited | Hospitality | 2022 |
| Osiria Limited | Hospitality | 2019 |
| Osotree Ltd | Construction | 2020 |
| Osteria Italian Ltd | Hospitality | 2018 |
| Oswalds Court (Property Management) Ltd | Administrative & support | 1998 |
| Oswestry Court House Limited | Construction | 2017 |
| Oswestry Walls Limited | Construction | 2016 |
| Oteley Gardens (Management Company) Limited | Real estate | 2020 |
| Ottolive Bakehouse Ltd | Manufacturing | 2014 |
| Ottolive Foods Ltd | Manufacturing | 2025 |
| OUR Own Wells Publishing Limited | Information & communication | 2024 |
| OUT of Africa Ltd | Wholesale & retail | 2018 |
| OUT of Bounds Immersive Experiences Ltd | Arts & recreation | 2025 |
| Outdoor Solutions (Shropshire) Ltd | Other services | 2025 |
| Outlaw Aviation Limited | Professional & scientific | 2025 |
| Outre MER Limited | Extraterritorial | 2004 |
| Outset Coffee Ltd | Hospitality | 2024 |
| Outsource Corporation Limited | Water & waste | 1997 |
| Over-Watch Group Ltd. | Administrative & support | 2012 |