| Octrim Software Limited | | 1986 |
| Oculas Installation Ltd | Construction | 2007 |
| Odiham RD Ltd | Financial services | 2022 |
| Odin IT Services Ltd | Information & communication | 2017 |
| Odyssey Global Limited | Administrative & support | 2003 |
| Office Champions Limited | Professional & scientific | 2003 |
| Offspring Rentals Ltd | Real estate | 2024 |
| Ogden Solutions Limited | Information & communication | 2008 |
| Oguns Limited | Information & communication | 1998 |
| Ohms House Limited | Real estate | 2020 |
| OIS Removals Limited | Transport & logistics | 2003 |
| OK Properties Limited | Real estate | 2003 |
| Okna Cleaning Solutions Ltd | Administrative & support | 2001 |
| Old Barn Court Residents Association Limited | Real estate | 2006 |
| Old Chislehurst Gate Management Limited | Households | 1988 |
| Old Coulsdon Bowling Club Limited | Arts & recreation | 1936 |
| Old Key Properties Ltd | Real estate | 2003 |
| Old School Cottages Management Company Limited | Households | 2004 |
| Old Square Management Limited | Real estate | 1987 |
| Old Stables Harrow Limited | Real estate | 2006 |
| Old Suttonians (Holdings) Limited | Administrative & support | 1979 |
| Old Tree Ltd | Hospitality | 2012 |
| Old Walcountians Limited | Extraterritorial | 1965 |
| Old Willow Ltd | Financial services | 1999 |
| Oleum Khaos Limited | Professional & scientific | 2004 |
| Oliver Hair Tech Limited | Other services | 1999 |
| Olivia Rickman Communications Ltd | Professional & scientific | 2018 |
| Om-Corporation Limited | Wholesale & retail | 2020 |
| Omar Mustafa Ltd | Transport & logistics | 2022 |
| Omead Investments Limited | Financial services | 2001 |
| Omni Airline Services Limited | Transport & logistics | 1998 |
| Omspine Limited | Wholesale & retail | 1982 |
| ON the River Limited | Real estate | 2018 |
| One Contract Services Holding Ltd | Real estate | 2019 |
| One Hundred and Seventeen Church Road (Richmond) Limited | Real estate | 1981 |
| One Roslyn Close Management Company Ltd. | Households | 1996 |
| One to Five Management Company Limited | Households | 1996 |
| One Way Design and Build Ltd | Construction | 2003 |
| Onecall Solutions Limited | Construction | 2007 |
| Onefourteen Limited | Transport & logistics | 2007 |
| Online Integrity Limited | Information & communication | 2004 |
| Online Merchandise Ltd | Information & communication | 2017 |
| Onswin Financial Services Ltd | Financial services | 2021 |
| Onswin Finserv Technology Limited | Information & communication | 2022 |
| Ontario Commerce Ltd | Wholesale & retail | 2019 |
| Opalrise Limited | Wholesale & retail | 1999 |
| Opaque Services Limited | Real estate | 2002 |
| Open Resources UK Limited | Information & communication | 2007 |
| Open Selling Limited | Professional & scientific | 2003 |
| Operandi Limited | Information & communication | 1996 |
| Operation Technical Consulting Ltd | Information & communication | 2025 |
| Opportune Art Services Limited | Arts & recreation | 1986 |
| Opsae Limited | Information & communication | 2018 |
| Optical Surfaces Limited | Manufacturing | 1991 |
| Optichrome Computer Solutions Limited | Professional & scientific | 1995 |
| Optichrome Computer Systems Limited | Information & communication | 1982 |
| Optichrome Group Limited | Financial services | 1996 |
| Optichrome Limited | Manufacturing | 1963 |
| Optichrome Properties Ltd | Real estate | 2016 |
| Optilabs (Croydon) Limited | Real estate | 2005 |
| Optimus Group Limited | Manufacturing | 2007 |
| Oraculum Ltd | Information & communication | 2006 |
| Orange Tree Theatre Limited | Arts & recreation | 1973 |
| Orca Media UK Limited | Information & communication | 2000 |
| Orchard (Elstead) Management Company Limited | Real estate | 2000 |
| Orchid Express Limited | | 1989 |
| Organizational and Educational Consultancy Limited | Administrative & support | 2003 |
| Origin Brand Consultants Limited | Professional & scientific | 1982 |
| Orion Pharm Limited | Wholesale & retail | 2024 |
| Orion Safety Consulting Limited | Professional & scientific | 2014 |
| Orlando Engineering Limited | Professional & scientific | 2007 |
| Oryx Development Ltd | Construction | 2026 |
| Osborne & Sons (Insurance Consultants) Limited | Financial services | 1963 |
| Osbornes Accountants Limited | Professional & scientific | 2002 |
| Osrap Estates Limited | Real estate | 2018 |
| Osteorom Limited | Health & social care | 2003 |
| Otherside Limited | Mining & quarrying | 2002 |
| OTI Capital Ltd | Financial services | 2024 |
| Outside the Box UK Limited | Arts & recreation | 2006 |
| Outsource Bookkeeping Centre Limited | Extraterritorial | 2006 |
| Ovaeda Limited | Construction | 2020 |
| Overcroft Limited | Administrative & support | 1992 |
| Overseas Personal Development Services Limited | Professional & scientific | 2002 |
| Overton Investments Limited | Real estate | 2005 |
| OVS Latham Ltd | Administrative & support | 2005 |
| Owen Properties (Bournemouth) Limited | | 1962 |
| Ownlife Fostering Ltd | Health & social care | 2005 |
| Oxford & London Electrical Ltd | Construction | 2019 |
| Oxley Accountants & Business Advisors Limited | Administrative & support | 2000 |
| Oxmed Health Limited | Health & social care | 2025 |
| Oxted Interiors Limited | Households | 2007 |
| Oxywell Limited | Arts & recreation | 2001 |
| OZ Bellikli Limited | Real estate | 2019 |
| O’Callaghan Property Limited | Real estate | 2025 |
| P & G Productions Limited | Information & communication | 2004 |
| P & U Mangal Limited | Health & social care | 2002 |
| P Gibbs Electrical Limited | Construction | 2006 |
| P Hayter Solutions Ltd | Information & communication | 2007 |
| P S Cabling Limited | Administrative & support | 2007 |
| P S Mcmillan Accounting Limited | Professional & scientific | 1991 |