| Optima Scaffolding Ltd | Administrative & support | 2011 |
| Optimal HR Solutions Ltd | Professional & scientific | 2020 |
| Optimize Works Ltd | Professional & scientific | 2025 |
| Optimum Nexus Ltd | Wholesale & retail | 2025 |
| Optimus Maximus Company Limited | Professional & scientific | 2003 |
| Optimus Point Ltd | Real estate | 2018 |
| Optxa Limited | Health & social care | 2021 |
| Oralyn Ltd | Wholesale & retail | 2025 |
| Orangered Limited | Information & communication | 2010 |
| Orangewood Homes Limited | Real estate | 2017 |
| Orasis Management Limited | Hospitality | 2014 |
| ORB. Nootropics Limited | Manufacturing | 2024 |
| Orbagel Limited | Hospitality | 2019 |
| Orbis Developments Ltd | Real estate | 2019 |
| Orbital Design Limited | Professional & scientific | 1999 |
| Orbital Labs Ltd | Information & communication | 2026 |
| Orchard & Co. Properties Limited | Real estate | 2025 |
| Orchard Court (Bournemouth) Limited | Households | 2013 |
| Orchard House Nursing Home Limited | Health & social care | 1996 |
| Orchard Plaza Management Company Limited | Real estate | 2007 |
| Ordinary Holdings Ltd | Professional & scientific | 2022 |
| Orestone Enterprise Ltd | Professional & scientific | 2011 |
| Organic Demand Ltd | Wholesale & retail | 2025 |
| Organica (LS) Limited | Manufacturing | 2023 |
| Organica (U.K) Limited | Wholesale & retail | 1998 |
| Organising Objects Ltd | Information & communication | 2026 |
| Organoid Intelligence Solutions Ltd | Information & communication | 2023 |
| ORH Ltd | Information & communication | 2023 |
| Oriel Court Property Ltd | Financial services | 2022 |
| Origin Yachts Ltd | Wholesale & retail | 2021 |
| Oriola Limited | Hospitality | 2021 |
| Orion Elements Limited | Wholesale & retail | 2020 |
| Orion Emporium Limited | Wholesale & retail | 2024 |
| Orion Warehouse Services Ltd | Transport & logistics | 2021 |
| Orionics Ltd | Information & communication | 2024 |
| Orler Brickwork Ltd | Construction | 2022 |
| ORM Consultants Ltd | Construction | 2018 |
| Ormer Holdings Ltd | Construction | 2024 |
| Orres Management Limited | Professional & scientific | 2009 |
| Ortho 2000 | Education | 1999 |
| Orthoshahin Ltd | Health & social care | 2024 |
| Orvylon Ltd | Wholesale & retail | 2025 |
| Orzo Holdings Limited | Financial services | 2023 |
| OS Technology Limited | Wholesale & retail | 2026 |
| Oscepal Ltd | Information & communication | 2021 |
| Oshy Limited | Financial services | 2016 |
| Osiris Property Ltd | Real estate | 2026 |
| Osma Properties Ltd | Real estate | 2020 |
| Osmington Estates Ltd | Real estate | 2025 |
| Osmond Group Limited | Wholesale & retail | 1962 |
| Osmosis Developers Limited | Real estate | 2026 |
| Osmund's House (Management) Limited | Households | 1983 |
| Osprey Accounting Limited | Professional & scientific | 2018 |
| Osprey Europe Limited | Wholesale & retail | 2003 |
| Osprey Retail Limited | Wholesale & retail | 2024 |
| OTA Corporation Limited | Professional & scientific | 2024 |
| OTA Recruitment Limited | Administrative & support | 2025 |
| Otto & Co Creative Ltd | Administrative & support | 2023 |
| Ounternet Limited | Wholesale & retail | 2024 |
| OUR Focus Ltd | Wholesale & retail | 2022 |
| OUR Gift Shop Limited | Wholesale & retail | 2026 |
| OUR Local Store Limited | Wholesale & retail | 2025 |
| Ourasphere Limited | Professional & scientific | 2023 |
| Ousadii Ltd | Education | 2022 |
| OUT and About Dorset Ltd | Health & social care | 2015 |
| OUT Real Estate Ltd | Real estate | 2021 |
| Outdoor Leisure Ltd | Wholesale & retail | 2024 |
| Outdoorgear Group Ltd | Wholesale & retail | 1948 |
| Outdoorgear Midlands Limited | Professional & scientific | 1927 |
| Outdoorgear Properties Ltd | Real estate | 1961 |
| Outdoorgear UK Limited | Wholesale & retail | 1999 |
| Outline Photos Ltd | Professional & scientific | 2025 |
| Outlook (Poole) Management Limited | Real estate | 2006 |
| Outreach Orbit Ltd | Information & communication | 2025 |
| Outrun Studios Ltd | Information & communication | 2019 |
| Outside IR35 Roles Limited | Information & communication | 2021 |
| OVC Ventures Limited | Financial services | 2024 |
| Overbury Manor (Management) Limited | Households | 1960 |
| Overexposure Sports Ltd | Arts & recreation | 2011 |
| Overhill Developments Ltd | Construction | 2024 |
| Overnight Worldwide Transportation and Logistics Ltd | Transport & logistics | 2021 |
| Overvue.Ai Ltd | Information & communication | 2024 |
| Ovingdean Limited | | 1997 |
| Ovittac Europe Limited | Real estate | 2020 |
| Owen Structural Limited | Professional & scientific | 2016 |
| Owens Estates Ltd | Real estate | 2026 |
| Ower Ltd | Administrative & support | 2020 |
| Owladi Limited | Real estate | 2005 |
| Ownix Limited | Wholesale & retail | 2021 |
| Ownlife Analytics Ltd | Professional & scientific | 2020 |
| Oxford Cricket Odyssey Ltd | Administrative & support | 2024 |
| Oxford Educational Services (UK) Limited | Construction | 2013 |
| Oxford Lettings and Holdings Limited | Real estate | 2025 |
| OXFORDDEDD01 Limited | Real estate | 2025 |
| Oxide Group Limited | Information & communication | 2023 |
| Oxor Ltd | Wholesale & retail | 2020 |
| Oxshop Ltd | Manufacturing | 2025 |
| Oxygen and Ozone Alliance Limited | Health & social care | 2026 |
| Oxygen Essential for Life Limited | Other services | 2026 |
| Oxygen Marketing Ltd | Other services | 2011 |