| One Step Professionals (Essex) Ltd | Health & social care | 2017 |
| One Stop 7 Days Ltd | Wholesale & retail | 2025 |
| One Stop Ecommerce Limited | Extraterritorial | 2009 |
| One Stop Scouting Limited | Wholesale & retail | 2006 |
| One to TEN Management Company Limited | Real estate | 2018 |
| One Trench Utilities Ltd | Energy | 2025 |
| One XEC Ltd | Administrative & support | 2019 |
| ONE77 Ltd | Information & communication | 2019 |
| ONE80 Smile Limited | Health & social care | 2014 |
| Onemoremobile Ltd | Wholesale & retail | 2019 |
| Onesys Financial Limited | Financial services | 2010 |
| Ongar Rugby Football Club Limited | Arts & recreation | 2015 |
| Online Professional Connections Ltd | Other services | 2020 |
| Onnest Care Service Limited | Administrative & support | 2026 |
| Onpoint Studios Ltd | Other services | 2023 |
| Onset Strength Ltd | Health & social care | 2023 |
| Onuklean Ltd | Administrative & support | 2022 |
| Onuks Building Improvement Services Ltd | Construction | 2022 |
| Onuksolutions Ltd | Construction | 2018 |
| Onyx Professionals Limited | Real estate | 2024 |
| Onyx Strength Ltd | Health & social care | 2024 |
| Oohso Limited | Professional & scientific | 2026 |
| Oops Details Ltd | Wholesale & retail | 2024 |
| OP Productions Ltd | Manufacturing | 2021 |
| Opal VA Services Ltd | Administrative & support | 2022 |
| Open Spaces Landscape and Arboricultural Consultants Limited | Agriculture & fishing | 2008 |
| Opennect Ltd | Information & communication | 2026 |
| Openwater Holdings Limited | Financial services | 2025 |
| Openwood Maintenance Limited | Construction | 2012 |
| Operating Systems Ltd | Professional & scientific | 2026 |
| Opes Shipping Limited | Wholesale & retail | 2013 |
| Oppidan (Apartments) Limited | Construction | 2015 |
| Oppidan (CL) Limited | Construction | 2013 |
| Oppidan (CL2) Limited | Construction | 2016 |
| Oppidan (GR) Limited | Construction | 2015 |
| Oppidan (KA) Limited | Construction | 2014 |
| Oppidan (Land Capital) Limited | Construction | 2012 |
| Oppidan (SR) Limited | Construction | 2020 |
| Oppidan Limited | Construction | 1999 |
| Opra Flex Ltd | Administrative & support | 2021 |
| Opra Group Ltd | Administrative & support | 2019 |
| Oprea Cargo Trans Ltd | Transport & logistics | 2019 |
| Optaflo Ltd | Information & communication | 2026 |
| Optevo Limited | Information & communication | 2025 |
| Optima Exchange Limited | Professional & scientific | 2019 |
| Optima Wholesalers Limited | Wholesale & retail | 2025 |
| Optimal IT Recruitment Ltd | Administrative & support | 2024 |
| Optimum Consultants Ltd | Other services | 2023 |
| Optimum Contractors Ltd | Construction | 2008 |
| Optimum Fabrication Ltd | Manufacturing | 2025 |
| Optimum Financial Advisors Limited | Financial services | 2002 |
| Optimum Fundraising Ltd | Wholesale & retail | 2022 |
| Optimum Reports Ltd | Information & communication | 2005 |
| Optimum Wealth Management LLP | | 2007 |
| Opulentcareservices Ltd | Professional & scientific | 2019 |
| Opusdawn Strategic Ltd | Real estate | 2025 |
| Orange County Projects Limited | Construction | 2022 |
| Orange Hampster Limited | Real estate | 2017 |
| Orange INK Consulting Limited | Information & communication | 2018 |
| Orange Lettings Ltd | Real estate | 2018 |
| Orange Pictures Ltd | Information & communication | 2018 |
| Orange Pipz Ltd | Information & communication | 2023 |
| Orange Tree Reserve Ltd | Administrative & support | 2025 |
| Orbit AV Limited | Information & communication | 2023 |
| Orchard Family Estates Limited | Real estate | 2005 |
| Orchard Hill Estates Limited | Households | 2023 |
| Orchid Rehabilitation Limited | Health & social care | 2018 |
| Oreste Construction Ltd | Construction | 2016 |
| Oriental Gogo Ltd | Hospitality | 2019 |
| Oriental Kitchen Chelmsford Ltd | Hospitality | 2025 |
| Origin Consultants Ltd | Professional & scientific | 2025 |
| Origin Renewable Energy Limited | Real estate | 2005 |
| Original Collection Ltd | Wholesale & retail | 2024 |
| Original Facades Limited | Construction | 2023 |
| Originalfire Limited | Construction | 2015 |
| Originals International Limited | Wholesale & retail | 2003 |
| Originstone & Critical Minerals Ltd | Mining & quarrying | 2026 |
| Oriol Associates Limited | Administrative & support | 2009 |
| ORL Medical Services Limited | Health & social care | 2021 |
| Orochi Ltd | Wholesale & retail | 2021 |
| Orsett Communications Limited | Other services | 2016 |
| Osborne Consultancy Ltd | Professional & scientific | 2024 |
| Oscar Fairchild Limited | Professional & scientific | 2002 |
| Oscar Muromba Enterprises Limited | Manufacturing | 2025 |
| Osea Products Limited | Professional & scientific | 2021 |
| OSH Commercial Ltd | Construction | 2026 |
| Osibisa Limited | Information & communication | 2022 |
| Osieye Healthcare Limited | Health & social care | 2020 |
| OSJ Fiducia Ltd | Other services | 2023 |
| OSJ Foundation Ltd | Health & social care | 2020 |
| Osmanq Limited | Construction | 2015 |
| Osobabs Care Service Ltd | Education | 2013 |
| OT Play Limited | Health & social care | 2013 |
| Otium Group Ltd | Real estate | 2004 |
| OTL Property Limited | Real estate | 2023 |
| Otna Infotech Ltd | Information & communication | 2024 |
| Otna Trading PVT Ltd | Wholesale & retail | 2026 |
| Otto's Family Farm Ltd | Agriculture & fishing | 2026 |
| Ouno App Ltd | Transport & logistics | 2019 |
| OUR Town Ltd | Information & communication | 2024 |