| Ocean Systems Limited | Real estate | 1984 |
| OCG Software Limited | Information & communication | 1981 |
| OCG Technology Ltd | Wholesale & retail | 2007 |
| Ockbrook Court Management Limited | Real estate | 1983 |
| Octaglobal Consulting Limited | Information & communication | 2024 |
| Octagon Clothing Ltd | Manufacturing | 2023 |
| Octagon Lincoln Ltd | Professional & scientific | 2024 |
| Octagon Technology Limited | Information & communication | 1997 |
| Octahedron Group Limited | Information & communication | 2021 |
| Odin Retail Holdings Limited | Financial services | 2006 |
| Odin Retail Limited | Wholesale & retail | 2005 |
| Odourfree Ltd | Administrative & support | 2026 |
| Odyssey Talent Management Ltd | Professional & scientific | 2026 |
| OE Digital Limited | Wholesale & retail | 2026 |
| OEM Parts Specialists Ltd | Wholesale & retail | 2024 |
| OFF the Wagon Mobile Bar Ltd | Hospitality | 2023 |
| Offgrid Collective Ltd | Wholesale & retail | 2025 |
| Office Canopy Group (Holdings) Limited | Wholesale & retail | 1998 |
| Officially Sourced Ltd | Professional & scientific | 2019 |
| Officii Limited | Administrative & support | 2019 |
| OFG Lincoln Ltd | Wholesale & retail | 2018 |
| OH Health Group Limited | Health & social care | 2022 |
| OH LA LA Streetfood Ltd | Hospitality | 2023 |
| OH Snap UK Limited | Arts & recreation | 2016 |
| OHL Group Limited | Households | 2026 |
| OHP Services Limited | Health & social care | 2016 |
| OIL & GAS Construction Services Ltd | Construction | 2009 |
| OJ Studio Services Ltd | Administrative & support | 2024 |
| Okavango Explorations (UK) Limited | Administrative & support | 1989 |
| Okebrook Ltd | Professional & scientific | 2014 |
| Okpemandu Limited | Health & social care | 2023 |
| Olax Electronics Limited | Wholesale & retail | 2014 |
| Old Angel Inn Limited | Hospitality | 2015 |
| Old Bakehouse Studios Limited | Real estate | 2021 |
| Old English Classics Limited | Extraterritorial | 2001 |
| Old Pond Development Management Limited | Real estate | 2025 |
| Old Wood Field Management Co Limited | Households | 2018 |
| Oldham Medical Ltd | Health & social care | 2026 |
| OLDPAIN2GO Limited | Health & social care | 2018 |
| OLE OLE Lincoln Limited | Hospitality | 2017 |
| OLE OLE Tapas Bar and Restaurant Limited | Extraterritorial | 2013 |
| Oleg Entrepreneurs Company Ltd | Professional & scientific | 2025 |
| OLI Binns Property Group Ltd | Real estate | 2025 |
| Olickel Properties Ltd | Real estate | 2026 |
| Olivares Ltd | Hospitality | 2014 |
| Olive & Co Property Group Ltd | Real estate | 2024 |
| Olive Cleaning Services Limited | Administrative & support | 2024 |
| Olive Tree Care Solutions Ltd | Health & social care | 2025 |
| Oliver & Co Properties Ltd | Real estate | 2025 |
| Oliver Kayes Ltd | Health & social care | 2014 |
| Oliver Wiseman Ltd | Health & social care | 2010 |
| Oliver Woodland Farrier Services Ltd | Extraterritorial | 2021 |
| Olli Ingall Music Limited | Information & communication | 2026 |
| Ollosoft Ltd | Information & communication | 2026 |
| Olumik Ltd | Health & social care | 2022 |
| OM Performance Ltd | Education | 2026 |
| Omega Hydrotechnics Ltd | Other services | 2018 |
| Omex Nitrogen Limited | Manufacturing | 1991 |
| ON Point Archery Ltd | Arts & recreation | 2020 |
| ON Site Welding Services Ltd | Manufacturing | 2020 |
| Once Upon a Time Design (Cornhill) Limited | Wholesale & retail | 2025 |
| Once Upon a Time Design (Lincoln) Limited | Manufacturing | 2024 |
| Once Upon a Time Design Limited | Manufacturing | 2023 |
| One Degree Training & Coaching Ltd | Education | 2022 |
| One East Midlands | Other services | 2001 |
| One Eleven (Commercial Property) Limited | Real estate | 2002 |
| One Eliquids Ltd | Extraterritorial | 2021 |
| One Harmony Investments Limited | Construction | 2022 |
| One Hundred Stars Management Limited | Professional & scientific | 2019 |
| One MAN PAN MAN Ltd | Wholesale & retail | 2026 |
| One Sky Aviators Ltd | Administrative & support | 2006 |
| One Sourrce Limited | Wholesale & retail | 2020 |
| One Stop Fone Shop Ltd | Wholesale & retail | 2024 |
| One-Ent Ltd | Health & social care | 2015 |
| ONE6TH Limited | Information & communication | 2018 |
| Onebodyonefaith Ltd. | Other services | 1995 |
| Oneclickplanthire Limited | Information & communication | 2025 |
| Oneplace Lettings Ltd | Hospitality | 2024 |
| Onestop Security Systems Ltd | Administrative & support | 2022 |
| Onexrm Ltd | Information & communication | 2020 |
| Online Teetimes Limited | Administrative & support | 2006 |
| Only Exposure Ltd | Professional & scientific | 2023 |
| Only the Vape Chesterfield Ltd | Wholesale & retail | 2016 |
| Only the Vape Derby RD Ltd | Wholesale & retail | 2023 |
| Only the Vape Four Seasons Ltd | Wholesale & retail | 2023 |
| Only the Vape GR Ltd | Hospitality | 2022 |
| Onra Consulting Limited | Professional & scientific | 2022 |
| Onsetra Ltd | Information & communication | 2025 |
| Onsite Mechanical Solutions Ltd | Manufacturing | 2023 |
| Onstream Management Systems Limited | Professional & scientific | 2003 |
| Onyx Trans Services Ltd | Transport & logistics | 2025 |
| Opalburger Ltd | Hospitality | 2025 |
| Opalence Realty Ltd | Real estate | 2025 |
| Opaltouch Services Ltd | Administrative & support | 2025 |
| Open Door (1908) Limited | Real estate | 2023 |
| Open Plan Consultants Ltd. | Professional & scientific | 2005 |
| Open Windows and Doors Ltd | Construction | 2021 |
| Open-Air Events Limited | Hospitality | 2016 |
| Operia Limited | Health & social care | 2019 |
| Opexcell Limited | Professional & scientific | 2025 |