| O'Keeffe Holdings Limited | Wholesale & retail | 2015 |
| O'Keeffe Measurement Limited | Wholesale & retail | 2018 |
| O.D. Briggs Consult Limited | Professional & scientific | 2022 |
| O.K.A.C Ltd | Hospitality | 2023 |
| O.S.S Global Solutions Ltd | Information & communication | 2023 |
| O365CONSULTANTS Limited | Professional & scientific | 2020 |
| Oak Foodstuff Trading Co. Ltd | Wholesale & retail | 2025 |
| Oak Hill BIO Holdings Ltd | Professional & scientific | 2021 |
| Oak Lane Investor Management Ltd | Real estate | 2016 |
| Oak Trees Financial Services Limited | Administrative & support | 2024 |
| Oakcastle Investments Limited | Real estate | 2012 |
| Oakesuk Group Limited | Financial services | 2013 |
| Oakesuk Limited | Manufacturing | 2020 |
| Oakfield Autos Limited | Wholesale & retail | 2012 |
| Oakfield Creative Investments Ltd | Real estate | 2020 |
| Oakfield Leasing & Finance Ltd | Financial services | 2009 |
| Oakfield Leasing Ltd | Financial services | 2024 |
| Oakfield Psychological Services Limited | Health & social care | 2013 |
| Oakhurst Evans Limited | Extraterritorial | 2022 |
| Oakland Commercial Limited | Real estate | 2011 |
| Oakland Residential Management Limited | Real estate | 2011 |
| Oaklands Estates (Sale) Limited | Construction | 2024 |
| Oakmere Design and Build Ltd | Construction | 2026 |
| Oakmere Kafe Ltd | Hospitality | 2026 |
| Oaknewmedia Limited | Professional & scientific | 2025 |
| Oakwood Corporate Nominees 2024 Limited | Professional & scientific | 2024 |
| Oakwood Corporate Nominees 2026 Limited | Professional & scientific | 2026 |
| Oakwood Corporate Secretary Limited | Administrative & support | 2009 |
| Oakwood Corporate Services Limited | Administrative & support | 2009 |
| Oakwood EMI Nominees Limited | Extraterritorial | 2022 |
| Oakwood Lane Properties Ltd | Real estate | 2018 |
| Oakwood Services (Altrincham) Ltd | Administrative & support | 2015 |
| Oakwood Trustees Limited | Professional & scientific | 2021 |
| Oakwood Valuation Surveyors Ltd | Professional & scientific | 2003 |
| Oasis International Foods Ltd | Wholesale & retail | 2020 |
| Obdm Ltd | Professional & scientific | 2015 |
| Obelisk Associates Limited | Professional & scientific | 2024 |
| Objml Limited | Professional & scientific | 2023 |
| Obsidian Automotive Limited | Wholesale & retail | 2022 |
| Obsidian Fire Limited | Professional & scientific | 2026 |
| Obsidian Innovation Limited | Financial services | 2022 |
| OC CMC Solutions and Scientific Training Ltd | Professional & scientific | 2025 |
| Ocean 1978 Ltd | Wholesale & retail | 2022 |
| Ocean Training and Interpreting Services Limited | Health & social care | 2021 |
| OCI Global Procurement Ltd | Wholesale & retail | 2025 |
| OCI OPS Limited | Wholesale & retail | 2025 |
| Oclegano Ltd | Transport & logistics | 2019 |
| OCP Media Ltd | Information & communication | 2020 |
| Octane Mortgages Ltd | Financial services | 2025 |
| Octant BIO UK Ltd | Manufacturing | 2025 |
| Octo Management Company Limited | Real estate | 2009 |
| Octopus Estates Limited | Real estate | 2018 |
| Ocular Consulting Ltd | Health & social care | 2018 |
| OCW Group Ltd | Wholesale & retail | 2025 |
| ODD Walk Studios Limited | Professional & scientific | 2023 |
| Odema Limited | Manufacturing | 2005 |
| Oden Technologies Ltd. | Information & communication | 2014 |
| Odin Bidco Limited | Financial services | 2021 |
| Odin Holdco Limited | Financial services | 2021 |
| Odin Midco Limited | Financial services | 2021 |
| Odin Topco Limited | Financial services | 2021 |
| Odontologist Limited | Health & social care | 2026 |
| Odulp Limited | Administrative & support | 2021 |
| Odyconsult Ltd | Administrative & support | 2024 |
| OEG II Limited Partnership | | 2008 |
| OEG Limited Partnership | | 2008 |
| OET Group Holdings Limited | Financial services | 2021 |
| OFF the High Street Limited | Wholesale & retail | 2013 |
| Offas Dyke Escape Ltd | Hospitality | 2021 |
| Office & Retail Services Ltd | Construction | 2000 |
| Office Blokes Ltd | Information & communication | 2020 |
| Office Collective Limited | Administrative & support | 2018 |
| Office Insight Holdings Ltd | Financial services | 2020 |
| Office Insight Limited | Construction | 2010 |
| Offsite Construction Developments Limited | Construction | 2024 |
| OG Analytics Limited | Administrative & support | 2018 |
| Ogecolnfalls Ltd | Transport & logistics | 2019 |
| OH My Home Ltd | Construction | 2021 |
| OH SIS Limited | Other services | 2025 |
| OH! Creative Studio Ltd | Professional & scientific | 2025 |
| OHB DME Ltd | Professional & scientific | 2022 |
| OHB Neonatology Ltd | Professional & scientific | 2022 |
| OHB Pediatrics Ltd | Professional & scientific | 2024 |
| OJB Forklift Training Limited | Other services | 2022 |
| OJK Accounting Solutions Ltd | Professional & scientific | 2020 |
| OJW Associates Ltd | Administrative & support | 2016 |
| Okehampton Eyecare Limited | Wholesale & retail | 2020 |
| Okehoe Limited | Professional & scientific | 2023 |
| Okosa Ltd | Wholesale & retail | 2022 |
| OLA Build Limited | Real estate | 2022 |
| Olam Group Ltd | Administrative & support | 2025 |
| Olam Trading Limited | Information & communication | 2017 |
| Olam.I Ltd | Health & social care | 2026 |
| OLB Investment & Asset Management Company Limited | Real estate | 2022 |
| Old Hall Court (Sale) Limited | Households | 1965 |
| Old Hulmeians Playing Fields Limited | Arts & recreation | 1924 |
| Old Meadows Fraser Homes Limited | Construction | 2024 |
| Old Meadows Management Co Limited | Households | 2024 |
| Old Shire Farm Foods Limited | Wholesale & retail | 2002 |
| Oldfield Estate Company Ltd | Real estate | 1999 |