| O'Rourke Property Management Ltd | Real estate | 2025 |
| O.N.M Joinery Ltd | Construction | 2020 |
| OA Mechs Ltd | Wholesale & retail | 2024 |
| Oak & Prosper (Glasshouses) Ltd | Construction | 2025 |
| Oak & Prosper (Investments) Limited | Construction | 2023 |
| Oak & Prosper (Oakwood) Ltd | Construction | 2022 |
| Oak & Prosper Construction Limited | Construction | 2016 |
| Oak & Prosper Harrogate Limited | Construction | 2025 |
| Oak Flooring & Beams Limited | Wholesale & retail | 2024 |
| Oak Property Trading Limited | Construction | 2018 |
| Oak Quarry Limited | Real estate | 2009 |
| Oakbridge Properties Limited | Real estate | 2018 |
| Oakcrest Joinery Ltd | Construction | 2022 |
| Oakdene Home Surveys Ltd | Professional & scientific | 2023 |
| Oakey Cokey Ltd | Manufacturing | 2023 |
| Oakjem Property Ltd | Real estate | 2025 |
| Oakland Dream Landscapes Ltd | Administrative & support | 2019 |
| Oakland House Residents Company Limited | Households | 1999 |
| Oaklands Property Solutions Ltd | Construction | 2023 |
| Oaklane Homeware Ltd | Extraterritorial | 2026 |
| Oakley Builders & Groundwork Contractors Ltd | Construction | 2011 |
| Oakley Enterprises Limited | Wholesale & retail | 2008 |
| Oakley's Home Improvement's Group Ltd | Construction | 2025 |
| Oakmont & Sons Ltd | Construction | 2026 |
| Oakridge Digital Ltd | Extraterritorial | 2025 |
| Oakroot Culinary Brands Ltd | Extraterritorial | 2025 |
| Oakroyd Homes Limited | Real estate | 2020 |
| Oaks Electrical & Renewables Ltd | Construction | 2026 |
| Oakspire Nexus Ltd | Extraterritorial | 2026 |
| Oakspire Ventures Ltd | Extraterritorial | 2026 |
| Oaktree Holiday Park & Fishery Limited | Hospitality | 2025 |
| Oakview Medical Ltd | Health & social care | 2011 |
| Oakwell Manufacturing Limited | Extraterritorial | 2026 |
| Oakwood Contracts Limited | Manufacturing | 1992 |
| Oakwood Dog Day School Ltd | Other services | 2015 |
| Oakwood Garden Services Ltd | Administrative & support | 2013 |
| Oasis Avenue Ltd | Hospitality | 2024 |
| Oasis Health & Fitness Ltd | Arts & recreation | 2016 |
| Oasis Leisure Products Limited | Manufacturing | 1990 |
| Oasis Leisure Services Ltd | Financial services | 2023 |
| Oats Developments Limited | Real estate | 2003 |
| Oberon Holdings Co Limited | Hospitality | 2018 |
| Obimedics Limited | Health & social care | 2024 |
| OBO Holding Limited | Financial services | 2022 |
| Obreon Limited | Information & communication | 2025 |
| OBS Property Management Ltd | Construction | 2026 |
| Obsidian Gate Ltd | Extraterritorial | 2025 |
| Occasions Hall Ltd | Hospitality | 2023 |
| Occasus Ltd | Real estate | 2022 |
| Occupational Health Doncaster Limited | Health & social care | 2020 |
| Ocean Beach Limited | Other services | 2017 |
| Ocean Electrical/Swimming Pool Solutions Ltd | Construction | 2019 |
| Ocean Pearl Properties Ltd | Real estate | 2024 |
| Ocean Tech Engineering Consultants Ltd | Extraterritorial | 2015 |
| Oceanaire Company Limited | Real estate | 2022 |
| Oceanchase Limited | Extraterritorial | 2010 |
| Oceanflame Limited | Extraterritorial | 2012 |
| Oceanhigh Limited | Extraterritorial | 2012 |
| Oceanic Elegance Ltd | Manufacturing | 2025 |
| Oceanpost Limited | Extraterritorial | 2012 |
| Oceanscript Limited | Extraterritorial | 2012 |
| Oceanshare Limited | Extraterritorial | 2012 |
| Oceanstop Limited | Extraterritorial | 2012 |
| OCL Solicitors Limited | Professional & scientific | 2007 |
| Ocnl Limited | Financial services | 2018 |
| Octagon Bespoke Limited | Extraterritorial | 2009 |
| Octagon Commercial Limited | Extraterritorial | 2009 |
| Octagon Consulting (Yorkshire) Ltd | Professional & scientific | 2023 |
| Octagon Design and Marketing Limited | Professional & scientific | 1996 |
| Octavian Place (Aylesbury) Management Company Limited | Households | 2004 |
| ODB Health Limited | Health & social care | 2026 |
| Odge Ltd | Households | 2020 |
| ODH Training Ltd | Education | 2024 |
| ODW Digital Ltd | Information & communication | 2024 |
| Odyssey Motorhome Hire Limited | Administrative & support | 2024 |
| Ofathaigh Ltd | Hospitality | 2024 |
| Offbeat London Ltd | Arts & recreation | 2015 |
| Office for People Ltd | Professional & scientific | 2016 |
| Official Form Ltd | Professional & scientific | 2009 |
| Offtraxx Ltd | Wholesale & retail | 2020 |
| OG Homes & Property Ltd | Real estate | 2025 |
| OGB Development Limited | Professional & scientific | 2024 |
| OGB Management Limited | Professional & scientific | 2018 |
| Oglesby Driver Hire Ltd | Administrative & support | 2023 |
| OH SEW Simple Limited | Other services | 2016 |
| OH SO Fine Tattoo Ltd | Arts & recreation | 2023 |
| OH SO Sweet Treats Limited | Wholesale & retail | 2022 |
| Ohaire Management Services Ltd | Professional & scientific | 2024 |
| Ohibiscus Ltd | Manufacturing | 2024 |
| OHS Models Limited | Wholesale & retail | 2016 |
| OHT Properties Limited | Real estate | 2024 |
| Oira Ltd | Wholesale & retail | 2025 |
| OJ Boutique Ltd | Wholesale & retail | 2025 |
| OJA Fusion Limited | Wholesale & retail | 2025 |
| OJC Holdings Limited | Financial services | 2017 |
| OJH Enterprises Ltd | Construction | 2025 |
| OJH Investments Ltd | Real estate | 2024 |
| OJM Music Limited | Arts & recreation | 2019 |
| Ojmiddleton Limited | Arts & recreation | 2016 |
| OK 2727 Ltd | Hospitality | 2025 |