| Oliver Property Limited | Real estate | 2020 |
| Olly Integrated Services Ltd | Transport & logistics | 2026 |
| Olly Wickes Live Sound Ltd | Professional & scientific | 2019 |
| OLM Electrical Limited | Construction | 2019 |
| Olnu Holdings Ltd | Financial services | 2025 |
| Olnu Ltd | Health & social care | 2024 |
| Olton Bedrooms Limited | Manufacturing | 2007 |
| Olympia Electrical Ltd | Construction | 2021 |
| Olympus Fitout Contractors Ltd | Construction | 2021 |
| OM Vang Vang Ltd | Hospitality | 2025 |
| Om'Mic Ltd | Wholesale & retail | 2026 |
| Omarson Management Ltd | Real estate | 2025 |
| Omdsp Handyman London Ltd | Construction | 2018 |
| Omdurman Health Ltd | Health & social care | 2019 |
| OME Logic Ltd | Information & communication | 2025 |
| Omega Print & Signs Limited | Wholesale & retail | 2018 |
| Omer Kebab Ltd | Hospitality | 2025 |
| Omicron Design and Safety Engineering Limited | Administrative & support | 2013 |
| Ominicorp Limited | Wholesale & retail | 2025 |
| Omltb Enterprises Ltd | Wholesale & retail | 2023 |
| Omnia Limited | Construction | 2004 |
| Omnia Recruitment Limited | Administrative & support | 2004 |
| OMUA89 Limited | Health & social care | 2026 |
| OMV Dance and Fitness Ltd | Education | 2026 |
| ON Plumbing & Heating Services Ltd | Construction | 2022 |
| Onboard Marine Electrical Ltd | Construction | 2019 |
| One Call 247 Building Services Limited | Construction | 2016 |
| One Caprera Guest House Limited | Hospitality | 2018 |
| One Fifty One Limited | Extraterritorial | 2024 |
| One Hundred and Twenty Four College Road Limited | Hospitality | 1995 |
| One Less Footprint Limited | Professional & scientific | 2016 |
| One Point Hire and Sales Ltd | Administrative & support | 2019 |
| One Red DOT Ltd | Information & communication | 2016 |
| One Riga Terrace Limited | Households | 2025 |
| One White Wall Limited | Professional & scientific | 2017 |
| ONE2ONE Plastering Ltd | Education | 2026 |
| Onecity Group Limited | Construction | 2026 |
| Oneworld Music Group Ltd | Other services | 2025 |
| ONF Securities Limited | Real estate | 2020 |
| Onio ANT Care Services Limited | Health & social care | 2021 |
| Onnl Limited | Health & social care | 2022 |
| Onward Community Group Ltd. | Other services | 2004 |
| Onward Community Wellbeing Group Ltd | Other services | 2024 |
| OOT & Aboot Care Services Ltd | Health & social care | 2022 |
| Ooyoot Ltd | Manufacturing | 2018 |
| Open Doors International Language School | Education | 2001 |
| Open Fitness & Wellbeing Ltd | Health & social care | 2025 |
| Open Kamera CIC | Professional & scientific | 2024 |
| Open Ocean Enterprises Limited | Professional & scientific | 2011 |
| Open Social Care Limited | Information & communication | 2025 |
| Operation Enrichment CIC | Education | 2024 |
| Operation Hernia | Health & social care | 2010 |
| Ophenia Group Ltd | Construction | 2018 |
| Opie Developments Limited | Construction | 2021 |
| OPJ Investments Ltd | Real estate | 2025 |
| OPM RFC | Arts & recreation | 2014 |
| Oprish Ltd | Construction | 2014 |
| Opsat (UK) Ltd | Administrative & support | 2017 |
| Opsat SW Ltd | Administrative & support | 2025 |
| Opsat Training (UK) Ltd | Administrative & support | 2024 |
| Optical Civil Engineering Ltd | Construction | 2014 |
| Optimal Wellbeing Solutions Limited | Health & social care | 2012 |
| Optimality Limited | Professional & scientific | 2006 |
| Optimise Construction Ltd | Construction | 2024 |
| Optimum Waste Solutions Ltd | Water & waste | 2025 |
| Optimus Windows and Doors Ltd | Construction | 2022 |
| Options 4 Limited | Information & communication | 2007 |
| Options Outdoor Ltd | Wholesale & retail | 2022 |
| Options Property Management & Lettings Ltd | Households | 2012 |
| Opulence & Co Birmingham Limited | Other services | 2016 |
| Opus DEI Ltd | Wholesale & retail | 2022 |
| Opus Facilities Solutions Ltd | Administrative & support | 2026 |
| Oracle Publications Ltd | Information & communication | 2006 |
| Orange (SW) Ltd | Administrative & support | 2018 |
| Orange FOX Property Ltd | Construction | 2024 |
| Orange Manufacturing Solutions Limited | Professional & scientific | 2025 |
| Orange Sunset Limited | Wholesale & retail | 2017 |
| Orange Tree Media Ltd | Information & communication | 2011 |
| Orawizard Ltd | Information & communication | 2016 |
| Orbis Global Ltd | Wholesale & retail | 2019 |
| Orbital Distribution Ltd | Wholesale & retail | 2003 |
| Orca Advisory Ltd | Education | 2025 |
| Orcathletics Ltd. | Water & waste | 2019 |
| Orchard Cottage Contracting Ltd | Other services | 2025 |
| Orchard Court (North Tawton) Management Limited | Hospitality | 2004 |
| Orchard Farm Devon Limited | Agriculture & fishing | 2024 |
| Orcinus Accountants Limited | Professional & scientific | 2023 |
| Orderever Limited | Hospitality | 1990 |
| Ordi Holdings Limited | Financial services | 2024 |
| ORG Holdings Limited | Professional & scientific | 2012 |
| Oriental Fusion Ltd | Hospitality | 2025 |
| Oriental PL2 Ltd | Hospitality | 2025 |
| Origin Endpoint Consulting Ltd | Information & communication | 2023 |
| Orion-Digital Diagnostics Ltd | Health & social care | 2019 |
| Orlando Care Homes Ltd | Health & social care | 2020 |
| Ormrod NON Exec Limited | Professional & scientific | 2022 |
| Ormrod Sales & Lettings Ltd | Real estate | 2021 |
| Ortava Outdoors Ltd | Education | 2026 |
| OS Creative (SW) Limited | Wholesale & retail | 2025 |
| Osas Global Services Limited | Construction | 2025 |