| Nutriba UK Limited | Manufacturing | 2020 |
| Nvet Holdings Ltd | Financial services | 2026 |
| Nylem AI Ltd | Information & communication | 2026 |
| Nzima Creations Ltd. | Manufacturing | 2020 |
| Oak Floors 4U Limited | Construction | 2013 |
| Oak IT Ltd | Information & communication | 2012 |
| Oak Tree Structured Finance Limited | Financial services | 2007 |
| Oak&Ivyuk Limited | Wholesale & retail | 2020 |
| Oaklab Limited | Information & communication | 2013 |
| Oakleigh Road Childcare Limited | Extraterritorial | 2019 |
| Oakrise Estates Ltd | Real estate | 2025 |
| Oakslade Ltd | Construction | 2005 |
| Oakslade Studios Ltd | Real estate | 2026 |
| Oaktree Court Limited | Health & social care | 1988 |
| Oakwood Financial Advisers Limited | Financial services | 2015 |
| Oakwood House Ltd | Real estate | 2006 |
| Oasis Property Rentals Limited | Real estate | 2002 |
| Oblivion Brake Systems Ltd | Manufacturing | 2024 |
| Obralink Limited | Professional & scientific | 2019 |
| Oceans ATE Alaska Limited | Information & communication | 2014 |
| Octagon Energy Ltd | Administrative & support | 2022 |
| Odyssey Sounds Ltd | Information & communication | 2025 |
| OEM Outdoor Limited | Professional & scientific | 2008 |
| OFF Grid Escapes Ltd | Hospitality | 2024 |
| Offa Tree Surgeons Limited | Administrative & support | 2013 |
| Oilquick UK Limited | Manufacturing | 2020 |
| OJH Properties Ltd | Real estate | 2025 |
| Oken Court Management Co. Limited | Households | 1980 |
| Oken Eyffler Limited | Real estate | 2013 |
| Old Fangled Limited | Manufacturing | 2022 |
| Old Potting Shed Lane Management Company Limited | Households | 2025 |
| Old Square Lettings Ltd. | Real estate | 2017 |
| Old Town Gardens (Stratford Upon Avon) Management Company Limited | Real estate | 2014 |
| Old Warwick Road Management Company Ltd | Households | 2019 |
| Olieve Ltd | Transport & logistics | 2018 |
| Olim Gifts Ltd | Wholesale & retail | 2021 |
| Olive Tree Properties Yorkshire Ltd | Real estate | 2023 |
| Oliveandcross Ltd | Wholesale & retail | 2026 |
| Oliver David Limited | Professional & scientific | 2016 |
| Omilia Limited | Professional & scientific | 2003 |
| Omoluabi Limited | Information & communication | 2020 |
| ON Centre Surveys Limited | Construction | 1973 |
| ON Point Installations Ltd | Construction | 2022 |
| ON Site Solutions Group Ltd | Construction | 2023 |
| ON the Spot Solutions Limited | Information & communication | 1996 |
| Onatti Productions Limited | Information & communication | 2009 |
| One Million Steps Limited | Administrative & support | 2013 |
| One Nation Studios Ltd | Information & communication | 2002 |
| One Neville Chambers Limited | Information & communication | 2007 |
| One Week Driving Specialist Limited | Other services | 2013 |
| ONE2TRACK UK Ltd | Wholesale & retail | 2025 |
| Onebus Limited | Transport & logistics | 2010 |
| Oneflow England Limited | Information & communication | 2022 |
| Oneschool Global UK | Education | 2017 |
| Oneschool Global UK CAP Limited | Education | 2020 |
| Oneschool Global UK Direct Ltd | Hospitality | 2025 |
| Opas Guides Limited | Manufacturing | 2022 |
| Open Exhibitions Limited | Construction | 2007 |
| Open Line Vitaly Limited | Information & communication | 2020 |
| Open Prepaid Funerals Limited | Other services | 2003 |
| OPEN2INNOVATE Ltd | Professional & scientific | 2025 |
| Opeongo Capital Management Limited | Financial services | 2024 |
| Ophelia Forbes Property Management Limited | Real estate | 2021 |
| Oppi Limited | Information & communication | 2022 |
| Optfour Limited | Extraterritorial | 2016 |
| Optima Engineering (Essex) Limited | Professional & scientific | 2017 |
| Optimal Estates Ltd | Households | 2021 |
| Optimization Direct Limited | Wholesale & retail | 2015 |
| Optimum Coaching and Consultancy Limited | Professional & scientific | 2013 |
| Optishake Ltd | Wholesale & retail | 2025 |
| Optiv Limited | Administrative & support | 2020 |
| Orange Tree Accounting Limited | Professional & scientific | 2004 |
| Orange Trolley Ltd | Transport & logistics | 2017 |
| Orbiz Emea Limited | Professional & scientific | 2018 |
| Orca Marine Leisure Group Limited | Professional & scientific | 2019 |
| Orca Marine Leisure Limited | Professional & scientific | 2007 |
| Orchard Close Tysoe Limited | Extraterritorial | 2015 |
| Orchard Rose Properties Limited | Real estate | 1996 |
| Orchid Catering Ltd | Hospitality | 2025 |
| Ordinary Applications Ltd | Information & communication | 2024 |
| Organise My Home Ltd | Professional & scientific | 2026 |
| Orient Warwick Limited | Hospitality | 2015 |
| Orobics Limited | Health & social care | 2014 |
| Orourkes Limited | Professional & scientific | 2008 |
| Orthopaedia Ltd | Professional & scientific | 2012 |
| Osborne Properties (Banbury) Ltd | Real estate | 2021 |
| Oscat Media Ltd | Professional & scientific | 2010 |
| Oscava Limited | Professional & scientific | 2018 |
| OSL UK PVT Ltd | Information & communication | 2020 |
| Oslo Corp UK Ltd | Extraterritorial | 2026 |
| Otis and Sons Ltd | Arts & recreation | 2024 |
| Otsf LLP | | 2024 |
| Otto Engineering Limited | Wholesale & retail | 2019 |
| OUR Living Place Ltd | Other services | 2026 |
| OUR Wellness Limited | Administrative & support | 2013 |
| Outdoor Creative Spaces Limited | Wholesale & retail | 2018 |
| Outdoor Home Improvements Ltd | Administrative & support | 2022 |
| Outrigo UK Limited | Professional & scientific | 2024 |
| Outsiders Music Publishing Limited | Information & communication | 2012 |
| Outsiders Recorded Music Limited | Information & communication | 2012 |