| Online Trading Sales Company Limited | Wholesale & retail | 2022 |
| Online Tuition Club Ltd | Education | 2024 |
| Only Business Solutions UK Limited | Administrative & support | 2022 |
| Only Scranz by TH Limited | Manufacturing | 2025 |
| Onsen Group Limited | Other services | 2021 |
| Onsen North Ltd | Real estate | 2025 |
| ONT Takeaway Ltd | Hospitality | 2024 |
| Onthemove Services Ltd | Transport & logistics | 2021 |
| Ontime Business Solutions Ltd | Information & communication | 2018 |
| Ontrak Community Initiative | Education | 2012 |
| ONX Media Limited | Information & communication | 2016 |
| OON Fabrics Ltd | Wholesale & retail | 2024 |
| OP - Gats. (PVT) Limited | Health & social care | 2013 |
| OPA Group Ltd | Wholesale & retail | 2023 |
| Opal Delight Catering Ltd | Manufacturing | 2024 |
| Opal Skin Atelier Ltd | Other services | 2025 |
| Opal VS Ltd | Wholesale & retail | 2021 |
| Opal Women's and Children's Hub CIC | Public administration | 2025 |
| Opalstone Realty Ltd | Real estate | 2025 |
| OPE Adire Ltd | Wholesale & retail | 2025 |
| Opel Services Ltd | Real estate | 2015 |
| Open Podiatry Limited | Administrative & support | 2016 |
| Open Road Cars Limited | Wholesale & retail | 2019 |
| Open Surgical Limited | Wholesale & retail | 2023 |
| Opening Boundaries CIC | Education | 2016 |
| Openmove Properties Limited | Real estate | 2016 |
| Opex Deployment Ltd | Professional & scientific | 2025 |
| Ophelia Graphics Limited | Professional & scientific | 2017 |
| Ophthalmic Choice Limited | Health & social care | 2014 |
| Opid Globals Limited | Wholesale & retail | 2025 |
| OPTERRA3 Ltd | Wholesale & retail | 2025 |
| Opti Global Resources Ltd | Wholesale & retail | 2018 |
| Opti Vita Private Limited | Wholesale & retail | 2024 |
| Optical Test & Calibration (UK) Limited | Professional & scientific | 2006 |
| Optical Test and Calibration (Holdings) Limited | Professional & scientific | 2023 |
| Optical Test and Calibration Limited | Professional & scientific | 1991 |
| Optiflow AI Ltd | Information & communication | 2026 |
| Optikal EYE Care Limited | Health & social care | 2006 |
| Optima Communication Limited | Information & communication | 2005 |
| Optima Eyecare Limited | Health & social care | 2010 |
| Optima Eyecare Locum Agency Ltd | Health & social care | 2015 |
| Optima Mortgages Limited | Financial services | 2023 |
| Optima Solutions IT Limited | Wholesale & retail | 2009 |
| Optimal Huddersfield Ltd | Hospitality | 2024 |
| Optimal Properties MR Limited | Real estate | 2018 |
| Optimal Recovery and Wellbeing Ltd | Education | 2024 |
| Optimas Marketing Ltd | Manufacturing | 2019 |
| Optimisation Developments Limited | Administrative & support | 2008 |
| Optimisation Investments Limited | Administrative & support | 2008 |
| Optimum Clean and Services Ltd | Administrative & support | 2026 |
| Optimum Guarding Ltd | Administrative & support | 2016 |
| Optimum Installations Limited | Mining & quarrying | 2002 |
| Optimum Legal Ltd | Other services | 2018 |
| Optimum Safety Systems Limited | Manufacturing | 1974 |
| Optimus GAS N Plumbing Services Ltd | Construction | 2022 |
| Options (Wibsey) Limited | Hospitality | 2016 |
| Optitrans Limited | Manufacturing | 2018 |
| Optiva Group Ltd | Health & social care | 2022 |
| Optivision Plus Limited | Health & social care | 2017 |
| Optivu Eyecare (Bradford) Ltd | Wholesale & retail | 2021 |
| Optiwatt Limited | Wholesale & retail | 2025 |
| Optocrom Ltd | Wholesale & retail | 2022 |
| Optomfx Ltd | Public administration | 2020 |
| Opulence Developments Limited | Real estate | 2005 |
| Opulence Executive Travel Limited | Wholesale & retail | 2015 |
| Opulence Motor House Limited | Wholesale & retail | 2024 |
| Opulence Suite Limited | Hospitality | 2023 |
| Opulent Edge Ltd | Real estate | 2025 |
| Opulent Land Developments Ltd | Real estate | 2016 |
| Opuluxemoments Ltd | Hospitality | 2024 |
| Opus Law Ltd | Extraterritorial | 2008 |
| Opusic Ltd | Wholesale & retail | 2023 |
| Oqsolutions Ltd | Professional & scientific | 2025 |
| OR & OR Limited | Real estate | 2021 |
| OR Transport Limited | Transport & logistics | 2025 |
| Orabeautyy Ltd | Wholesale & retail | 2025 |
| Oracle Optical Ltd | Other services | 2017 |
| Oracle Regale Ltd | Other services | 2024 |
| Oral Health & Wellness Ltd | Health & social care | 2026 |
| Oral Surgery and Implant Services Ltd. | Health & social care | 2020 |
| Orange Shop 33 Ltd | Wholesale & retail | 2025 |
| ORB Recruitment Ltd | Administrative & support | 2017 |
| Orbit Handicrafts Ltd | Professional & scientific | 2024 |
| Orbital Environmental Ltd | Construction | 2026 |
| Orbital Waste Solutions Ltd | Water & waste | 2025 |
| Orbitnest Ltd | Wholesale & retail | 2025 |
| Orchard Adolescent Care Group Ltd | Health & social care | 2023 |
| Orchard Living Ltd | Hospitality | 2019 |
| Orchid Textiles Ltd | Manufacturing | 2020 |
| Ordexa Ltd | Wholesale & retail | 2015 |
| Oreza Limited | Information & communication | 2024 |
| Organic Distribution Limited | Wholesale & retail | 2025 |
| Organic Mind Limited | Other services | 2015 |
| Organic Relief Ltd | Health & social care | 2020 |
| Orgheenic Ltd | Manufacturing | 2024 |
| Oriental Arts (Bradford) Limited | Arts & recreation | 1994 |
| Origin Import and Export Ltd | Wholesale & retail | 2021 |
| Original Abaseen Ltd | Hospitality | 2022 |
| Original Software Designs Limited | Transport & logistics | 2025 |
| Orion Interiors Properties Ltd | Real estate | 2022 |