| O'Neill Racing Ltd | Arts & recreation | 1989 |
| O. Bjelland Limited | Professional & scientific | 2002 |
| O.E.Drewitt Contracting Limited | Construction | 2017 |
| Oak Productions Ltd | Information & communication | 2023 |
| Oaken Acre Ltd | Real estate | 2025 |
| Oakhaven Consultancy Ltd | Professional & scientific | 2013 |
| Oaklands (Baughurst) Residents Company Limited | Households | 2005 |
| Oaklands Grove (Maidenhead) Management Company Limited | Households | 2020 |
| Oaklands Shareblock Limited | Real estate | 2003 |
| Oakleigh Club Limited | Other services | 2026 |
| Oakleigh Roofing Ltd | Construction | 2024 |
| Oakwood (Newbury) Management Company Limited | Households | 1984 |
| Oare Organics Limited | Administrative & support | 1998 |
| Oareborough Consulting Limited | Professional & scientific | 2012 |
| Oasis Hair Boutique Ltd. | Other services | 2018 |
| OBH Consulting Group Ltd | Professional & scientific | 2022 |
| Ocean Media Production Limited | Information & communication | 2023 |
| Ocean Spas Limited | Wholesale & retail | 2020 |
| Ockham Film Production Limited | Information & communication | 2017 |
| Octogenti Limited | Real estate | 2025 |
| Oculus Wealth Management (Newbury) Limited | Financial services | 2020 |
| ODD Fellows Court Management Company Limited | Households | 1998 |
| Oddfellows Services Limited | Information & communication | 2019 |
| Oddrock Consulting Limited | Professional & scientific | 2016 |
| Odin Technology Limited | Information & communication | 1999 |
| OEC Bidco Limited | Financial services | 2026 |
| OEC International Limited | Professional & scientific | 2014 |
| Oeconnection Limited | Information & communication | 1989 |
| OET Limited | Professional & scientific | 2006 |
| OFF the Block.Shop Ltd | Wholesale & retail | 2026 |
| OFF the Scale Catering Ltd | Hospitality | 2024 |
| Offbeat Films Ltd | Information & communication | 2019 |
| Office Clean UK Ltd | Extraterritorial | 2017 |
| Office Project Solutions Limited | Real estate | 2008 |
| Officium - BPO (UK) Limited | Other services | 2021 |
| Officium Delivery Ltd | Real estate | 2017 |
| Officium Digital Limited | Professional & scientific | 2024 |
| Officium Financial Limited | Professional & scientific | 2025 |
| Officium Growth Ltd | Information & communication | 2022 |
| Officium Homes (WB) Limited | Real estate | 2026 |
| Officium Legacy Ltd | Administrative & support | 2024 |
| Ogbourne Solutions Limited | Other services | 2017 |
| Ogica LLP | | 2019 |
| OH SOO Able to Label Ltd | Manufacturing | 2021 |
| OJ Automotive Ltd | Wholesale & retail | 2025 |
| Ojam Ltd | Transport & logistics | 2026 |
| OJB Creations Ltd | Professional & scientific | 2014 |
| Old Kingsclere Brewery Management Limited | Real estate | 2007 |
| Oldencars Ltd | Other services | 2018 |
| Olive and Oscar's Academy Ltd | Other services | 2023 |
| Olive Creative Limited | Arts & recreation | 2003 |
| Olivia Morgan Ltd | Wholesale & retail | 2016 |
| Olivia Simonazzi Europe Limited | Manufacturing | 2019 |
| Olympic Fixing Products Limited | Wholesale & retail | 1989 |
| Olympic Fixings Ireland Ltd | Wholesale & retail | 1991 |
| Omotive Ltd | Professional & scientific | 2026 |
| ON Demand Consulting & Education Ltd | Information & communication | 2010 |
| ON the MAT Fitness Limited | Arts & recreation | 2023 |
| One Across Ltd | Professional & scientific | 2024 |
| One Financial Adviser 2022 Limited | Financial services | 2021 |
| One Financial Adviser Ltd | Financial services | 2011 |
| One Good Sunday Ltd | Wholesale & retail | 2024 |
| One Stop Service Centres Ltd | Wholesale & retail | 1999 |
| Onenet Associates Limited | Administrative & support | 2010 |
| Onez Music Ltd | Arts & recreation | 2015 |
| ONLINE-4-U Ltd | Manufacturing | 2012 |
| Onlitech Limited | Information & communication | 2000 |
| Onosoooo Limited | Information & communication | 2023 |
| Onset Fire Limited | Construction | 2015 |
| Onsplash Limited | Administrative & support | 2012 |
| Onyx Commercial Limited | Wholesale & retail | 2012 |
| Open Book Digital Advertising UK Ltd | Professional & scientific | 2024 |
| Open Door Investment Ltd | Real estate | 2015 |
| Open IE Limited | Information & communication | 2024 |
| Open Metrics Limited | Information & communication | 2021 |
| OPI Solutions Limited | Information & communication | 2012 |
| Opleah Limited | Information & communication | 2021 |
| Opskin Ltd | Manufacturing | 2025 |
| Optimo Ventures Ltd | Real estate | 2003 |
| Optimus Safety Limited | Other services | 2014 |
| Optimux Ltd | Professional & scientific | 2012 |
| Options Energy Resource Limited | Financial services | 2024 |
| ORA Holdings Limited | Financial services | 2012 |
| Orange DOT Consulting Limited | Professional & scientific | 2014 |
| Orbis Industria Limited | Other services | 2021 |
| Orbitr Ltd | Information & communication | 2023 |
| Orchard Park Close Limited | Real estate | 2017 |
| Origin 24 Limited | Information & communication | 2024 |
| Origin Corporate Events Limited | Other services | 2015 |
| Origin Drinks Limited | Wholesale & retail | 2011 |
| Origin Engineering Consultancy Ltd | Professional & scientific | 2021 |
| Origin Hospitality Limited | Other services | 1997 |
| Orlando ELY Limited | Wholesale & retail | 2023 |
| Ortho Consulting Group Ltd | Professional & scientific | 2010 |
| Ortho Executive Limited | Other services | 2017 |
| Ortho Surgical Skills Centre Limited | Extraterritorial | 2024 |
| Orthocg Holdings Limited | Other services | 2021 |
| Orthocg Ltd | Other services | 2018 |
| Orthoconnections Ltd | Extraterritorial | 2011 |
| Ossc Limited | Extraterritorial | 2024 |