| Onecrest Ltd | Financial services | 2026 |
| Onefloc Limited | Other services | 2024 |
| Onesave Limited | Wholesale & retail | 2020 |
| Onetymecare and Environmental Consultancy Limited | Health & social care | 2015 |
| Onevo Limited | Professional & scientific | 2014 |
| Onkar Restaurant Ltd | Hospitality | 2022 |
| Onley Holdings Limited | Financial services | 2024 |
| Online ADS London Ltd | Professional & scientific | 2021 |
| Online Business Resources Ally Ltd | Other services | 2020 |
| Only Van Sales Ltd | Wholesale & retail | 2023 |
| Onlycars Ltd | Wholesale & retail | 2022 |
| Onlyvit Ltd | Manufacturing | 2026 |
| ONN Developments Limited | Construction | 2025 |
| Onofri & Ortenzi Food Company Ltd | Hospitality | 2025 |
| Onply Ltd | Wholesale & retail | 2026 |
| Onpoint Handyman Limited | Construction | 2025 |
| Onroute Taxis Ltd | Transport & logistics | 2025 |
| Ontapevents Ltd | Hospitality | 2022 |
| Ontheroad Ltd | Transport & logistics | 2025 |
| Onward Path Care Limited | Hospitality | 2021 |
| Onward Treatment Ltd | Other services | 2023 |
| Onyde Ltd | Professional & scientific | 2018 |
| Onyeka Gabriel IBE Ltd | Health & social care | 2021 |
| Onyx Diggers UK Ltd | Wholesale & retail | 2023 |
| Onyx Digital Limited | Professional & scientific | 2025 |
| OP Communications Ltd | Information & communication | 2017 |
| Opam Services Limited | Professional & scientific | 2023 |
| Open Concept Ltd | Construction | 2022 |
| Open Digital Web Ltd | Professional & scientific | 2016 |
| Opendoor Holdings Ltd | Professional & scientific | 2024 |
| Opendoor Investment Services Ltd | Financial services | 2024 |
| Opendoor Mortgages Limited | Financial services | 2021 |
| Opendoor Rentals Limited | Real estate | 2021 |
| Openfaas Ltd | Information & communication | 2017 |
| Operatives One Ltd | Administrative & support | 2025 |
| Opharma Limited | Health & social care | 2014 |
| Ophtha Care Limited | Administrative & support | 2014 |
| Ophthalmic Consultants Limited | Health & social care | 2014 |
| Opieka Care Ltd | Education | 2015 |
| Opliukashgv Ltd | Transport & logistics | 2022 |
| Opsunity Ltd | Information & communication | 2025 |
| Opti Task Solutions Ltd | Administrative & support | 2025 |
| Optical AF Ltd | Wholesale & retail | 2025 |
| Opticz Limited | Administrative & support | 2023 |
| Optide Solutions Ltd | Construction | 2019 |
| Optilabx Ltd | Professional & scientific | 2025 |
| Optima Healthcare and Management Ltd | Information & communication | 2025 |
| Optima Supplies Limited | Wholesale & retail | 2026 |
| Optimal Energy Limited | Professional & scientific | 2017 |
| Optimise Multimedia Ltd | Information & communication | 2021 |
| Optimo Build Co Ltd | Construction | 2021 |
| Optimum Digits Academy Limited | Professional & scientific | 2025 |
| Optimum Digits Ltd | Professional & scientific | 2023 |
| Optimum Electrics Ltd | Construction | 2011 |
| Optimum Group Property Holdings Limited | Real estate | 2015 |
| Optimum Land and Property Limited | Real estate | 2011 |
| Optimum Lettings and Property Management Limited | Real estate | 2010 |
| Optimum Plumbing & Heating Limited | Construction | 2004 |
| Optimum Residential Sales Limited | Real estate | 2017 |
| Optimum Solar International Ltd | Energy | 2025 |
| Optimum Vehicle Group Ltd | Wholesale & retail | 2021 |
| Opulence Car Sales Ltd | Wholesale & retail | 2026 |
| Opulence Money Limited | Financial services | 2017 |
| Opulent Boutique Hotels Ltd | Hospitality | 2024 |
| Opulentia Tech Limited | Wholesale & retail | 2026 |
| Orange Accounting Ltd | Professional & scientific | 2022 |
| Orasis Limited | Health & social care | 2010 |
| ORB Advertising Ltd | Professional & scientific | 2023 |
| Orbis Investments Group Limited | Financial services | 2022 |
| Orbis Trading Limited | Financial services | 2013 |
| Orbit Removals Ltd | Transport & logistics | 2025 |
| Orcalab Ltd | Information & communication | 2025 |
| Orchard Farm Little Warley Limited | Construction | 2024 |
| Orchard Fields (Over) Residents Association Limited | Households | 1982 |
| Orchard Properties (Peterborough) Ltd | Real estate | 2021 |
| Orchid Bathrooms Ltd | Construction | 2008 |
| Order Ltd | Wholesale & retail | 2022 |
| Ordina Property Investment Ltd | Real estate | 2025 |
| Oregyn Group Ltd | Professional & scientific | 2016 |
| Orford Properties Limited | Households | 2005 |
| Organic Recycling Limited | Water & waste | 1995 |
| Oriam Etuh Lettings & Estate Ltd | Real estate | 2025 |
| Oriental Eats Ltd | Hospitality | 2025 |
| Origin Marketing Ltd. | Professional & scientific | 2013 |
| Orion 1977 Ltd | Transport & logistics | 2026 |
| Orion Made This Ltd | Wholesale & retail | 2025 |
| ORION5 Ltd | Professional & scientific | 2023 |
| Orison Financial Management Limited | Financial services | 2022 |
| Orlowski Trans Ltd | Transport & logistics | 2018 |
| Ormesby Road Flats Management Company Limited | Households | 2009 |
| Orodom Limited | Financial services | 2026 |
| Orofacial Restorations Ltd | Manufacturing | 2020 |
| Orsus-Uk Ltd | Real estate | 2020 |
| Orthopaedics and Spine Specialty Clinic Limited | Health & social care | 2000 |
| Orton Advisory Ltd | Professional & scientific | 2026 |
| Orton Contract Services Ltd | Professional & scientific | 2018 |
| Orton Decorating Contractors Limited | Construction | 1987 |
| Orton Pharma Ltd | Wholesale & retail | 2023 |
| Orton Phone Stop Ltd | Wholesale & retail | 2025 |
| Ortonvets Ltd | Professional & scientific | 2014 |