| Ontime Business Ltd | Wholesale & retail | 2020 |
| Ontrack Roofing & Building Ltd | Construction | 2025 |
| Onyx Synergy Limited | Professional & scientific | 2019 |
| Onyx Talent Team Culture Limited | Professional & scientific | 2023 |
| Oosc Clothing Limited | Manufacturing | 2015 |
| Opal Software Limited | Information & communication | 1998 |
| Opalguide Limited | Real estate | 2002 |
| Open College UK Limited | Education | 2005 |
| Open GI London Limited | Information & communication | 1998 |
| Open Insurance Management Limited | Information & communication | 1992 |
| Open International Limited | Information & communication | 2006 |
| Open Power Limited | Information & communication | 1986 |
| Open Strategy Limited | Professional & scientific | 2006 |
| Openreward Limited | Professional & scientific | 1991 |
| Opfc Ltd | Professional & scientific | 2024 |
| Opia Aesthetics Ltd | Other services | 2021 |
| OPN Trade Ltd | Wholesale & retail | 2026 |
| Opsentric Operational Intelligence Ltd | Hospitality | 2026 |
| Opsergo Limited | Professional & scientific | 2024 |
| Opticomms West Midlands Ltd | Information & communication | 2024 |
| Optimise Energy UK Ltd | Professional & scientific | 2017 |
| Optimus Associates Limited | Professional & scientific | 1994 |
| Opulence(Uk) Ltd | Education | 2017 |
| Opulent Landscape and Driveways Ltd | Administrative & support | 2022 |
| Opus Autoroute Limited | Administrative & support | 2014 |
| Opus Biological Limited | Health & social care | 2018 |
| Opus Labs Limited | Professional & scientific | 2021 |
| Opus Software Automation Development Limited | Information & communication | 2016 |
| Orange Cactus Limited | Real estate | 2018 |
| ORB Sfeer CIC | Professional & scientific | 2025 |
| Orbit Portsmouth Limited | Real estate | 2002 |
| Orca Professional Services Ltd | Construction | 2015 |
| Orchard Assessments Ltd | Construction | 2015 |
| Orchard Consultancy Solutions Ltd | Professional & scientific | 2017 |
| Orchard Leigh Developments Limited | Construction | 2002 |
| Orchard Vine Limited | Professional & scientific | 2017 |
| Orchid Resources Ltd | Other services | 2025 |
| Orchids Thai Restaurant Ltd | Hospitality | 2000 |
| Organic Ecommerce Limited | Information & communication | 2017 |
| Organic Trading Limited | Wholesale & retail | 2020 |
| Orient Foods Limited | Wholesale & retail | 2017 |
| Oriental Bazaar Ltd | Extraterritorial | 2008 |
| Oriental Health Spa Ltd | Health & social care | 2024 |
| Origin Arts Limited | Arts & recreation | 2011 |
| Original Edge Media Limited | Other services | 2014 |
| Original Go Creative Limited | Professional & scientific | 2021 |
| Original Image Media Concepts Limited | Information & communication | 2003 |
| Origins Sound Ltd | Information & communication | 2014 |
| ORJ Construction Ltd | Construction | 2024 |
| Ormlee Ltd | Real estate | 2024 |
| Ornamental Tarantulas Ltd | Extraterritorial | 2026 |
| Orocca Limited | Other services | 2022 |
| Ortega Car Repairs Ltd | Wholesale & retail | 2018 |
| Orthocare Clinic Ltd | Health & social care | 2025 |
| Orthopaedic Academy Ltd | Education | 2020 |
| Orthopaedic Sports Medicine Limited | Health & social care | 2011 |
| Osbornes Architects Limited | Professional & scientific | 2025 |
| Ostlers at Number One Ltd | Hospitality | 2018 |
| OTB Designs Ltd | Real estate | 2023 |
| Otherton Farm Dog Park Ltd | Other services | 2023 |
| OTL Group Ltd | Transport & logistics | 2018 |
| Ottwood Limited | Real estate | 2025 |
| Oulsnam Limited | Hospitality | 2025 |
| Oulton Properties Limited | Real estate | 1996 |
| Ounces Bar and Restaurant Limited | Hospitality | 2018 |
| OUR Healthy World Ltd | Wholesale & retail | 2023 |
| OUR Place (2024) Limited | Financial services | 2024 |
| OUR Space Play & Family Therapies CIC | Health & social care | 2023 |
| Ouse TPI Limited Partnership | | 2018 |
| OUT 2 WIN Limited | Real estate | 2019 |
| OUT FOX IT Ltd | Information & communication | 2022 |
| Outerclo Limited | Wholesale & retail | 2024 |
| Outfox SEO Ltd | Other services | 2015 |
| Outpost Developments Ltd | Construction | 2021 |
| Outsource Workforce Ltd | Administrative & support | 2026 |
| Outwin Engineering and Manufacturing Limited | Professional & scientific | 1976 |
| OV Talent Limited | Professional & scientific | 2023 |
| Ovantis Limited | Financial services | 2025 |
| Ovastone Limited | Real estate | 2010 |
| Overt Software Solutions Limited | Information & communication | 2010 |
| Owen Pabari Marchment LLP | | 2009 |
| Owens Consulting ( UK ) Limited | Administrative & support | 2009 |
| Oxbridge Developments Limited | Professional & scientific | 1997 |
| Oxford Hardware Limited | Wholesale & retail | 2001 |
| Oxford Management Consultancy Ltd | Information & communication | 1999 |
| Ozac Holdings Ltd | Financial services | 2024 |
| Ozzy Gaffa Ltd | Hospitality | 2025 |
| P & J Inclusive Leadership Company Ltd | Professional & scientific | 2021 |
| P & R Alarms Limited | Administrative & support | 2001 |
| P & R Fire & Security Limited | Administrative & support | 2006 |
| P and D (Worcester) Limited | Construction | 2018 |
| P Birdsall Couriers Ltd | Transport & logistics | 2016 |
| P C Anning Limited | Professional & scientific | 2024 |
| P G Worcester Ltd | Real estate | 2019 |
| P Grove Limited | Construction | 2005 |
| P Hill Care Limited | Health & social care | 2020 |
| P Holliday Consulting Ltd | Professional & scientific | 2025 |
| P J Fabrications and Sheetmetal Limited | Professional & scientific | 2021 |
| P J Kitching Ltd | Construction | 2021 |
| P Marsh Consulting Limited | Information & communication | 2016 |