| Nutbrown Consultants Limited | Professional & scientific | 2012 |
| Nutrient Dense Food Limited | Extraterritorial | 2014 |
| Nutrition Mentoring Ltd | Health & social care | 2019 |
| NWD Logistics Ltd | Administrative & support | 2021 |
| NY Finance Limited | Professional & scientific | 2023 |
| O'Connor Group Holdings Limited | Real estate | 2019 |
| O. M & K Consultants Limited | Professional & scientific | 2013 |
| O.J.G Properties Ltd | Real estate | 2025 |
| O.W. Groundworks & Landscapes Ltd | Construction | 2024 |
| O3 MTC Property Ltd | Real estate | 2021 |
| OAI (Leics) Holdings Ltd | Financial services | 2024 |
| Oak and ASH Garden Services Ltd | Administrative & support | 2010 |
| Oak Grange (Littleover) Management Company Limited | Households | 2020 |
| Oak Inspirations Limited | Construction | 2007 |
| Oak Shields Estate Planning Limited | Professional & scientific | 2021 |
| Oakdale Restorations Limited | Other services | 2018 |
| Oaks + Land Ltd | Real estate | 1996 |
| Oakstone Facilities Ltd | Administrative & support | 2026 |
| Oakwolf Properties Ltd | Real estate | 2020 |
| Oakwood Capital Wealth Management Limited | Financial services | 2023 |
| Oakwood Manor Care Home Ltd | Health & social care | 2025 |
| Oasis Hair & Beauty (Leics) Limited | Other services | 2012 |
| Oathbrook Limited | Real estate | 1981 |
| OB Decorators Ltd | Construction | 2019 |
| Obsidian Link Ltd | Information & communication | 2019 |
| Ocean Blue Melton Mowbray Limited | Hospitality | 2023 |
| Ocmus Ltd | Construction | 2022 |
| Octane Photographic Ltd | Administrative & support | 2010 |
| Octavian Transport Ltd | Transport & logistics | 2015 |
| October Rose Limited | Other services | 2018 |
| OCV Products Ltd | Manufacturing | 2024 |
| Odin Site Services Ltd | Manufacturing | 2022 |
| Odontech Ltd | Manufacturing | 2022 |
| ODS Property Ltd | Real estate | 2021 |
| Odu-Uk Limited | Wholesale & retail | 1998 |
| OFF Leash Ltd | Wholesale & retail | 2020 |
| OFF Piste Solutions Ltd | Professional & scientific | 2025 |
| Office Village Limited | Financial services | 2026 |
| Offon Limited | Wholesale & retail | 2014 |
| OGC Enterprises Limited | Real estate | 2024 |
| Ogle Street Ventures Limited | Real estate | 2024 |
| OHH Deer Ltd | Wholesale & retail | 2011 |
| OJ Sparkle Services Limited | Administrative & support | 2026 |
| OJD Cars Ltd | Wholesale & retail | 2026 |
| Olaf Environmental Health Consultancy Ltd | Professional & scientific | 2022 |
| Old Bull Farm Management Limited | Extraterritorial | 2004 |
| Old Custard Ltd | Hospitality | 2026 |
| Old VM Investments Company Ltd | Real estate | 2019 |
| Olympus Property Ventures Limited | Real estate | 2025 |
| Omega Plumbing and Heating Limited | Construction | 2005 |
| Omita Ltd | Administrative & support | 2022 |
| Omnia Holistic Clinic Ltd | Health & social care | 2020 |
| ON the Bay Transport Ltd | Transport & logistics | 2021 |
| ON365 Limited | Manufacturing | 1990 |
| Oncology Care Limited | Health & social care | 2023 |
| Oncotherics Ventures Limited | Professional & scientific | 2020 |
| Onda Digital Limited | Information & communication | 2024 |
| One Akaal Investments Ltd | Financial services | 2023 |
| One PAW at a Time Ltd | Other services | 2019 |
| One Stop Promotions Limited | Wholesale & retail | 2005 |
| One Take Socials Ltd | Information & communication | 2024 |
| Oneclick Platform UK Ltd | Information & communication | 2016 |
| Onetaxresource Limited | Extraterritorial | 2014 |
| Onit Golf Ltd | Manufacturing | 2024 |
| Online Giving Ltd | Information & communication | 2009 |
| Online Insulation Ltd | Wholesale & retail | 2015 |
| Onpoint Performance Ltd | Manufacturing | 2024 |
| Onpoint Property Care Limited | Construction | 2024 |
| Onsite Commercial Cleaners Limited | Administrative & support | 2021 |
| Onthethird Ltd | Wholesale & retail | 2026 |
| Ontic Psychology Ltd | Health & social care | 2024 |
| Onyx Fire Services Limited | Public administration | 2023 |
| Oosha Limited | Professional & scientific | 2009 |
| OPC Maintenance Limited | Construction | 2026 |
| Open Box Promotions Limited | Information & communication | 2010 |
| Open Heaven Church | Other services | 2010 |
| Opnote Ltd | Information & communication | 2022 |
| Optabi Limited | Information & communication | 2018 |
| Optimal Hearing Limited | Wholesale & retail | 2024 |
| Optimum Mental Health and Wellbeing Ltd | Health & social care | 2024 |
| Optimum Path Consulting Limited | Professional & scientific | 2026 |
| Optimum Path Group Ltd | Financial services | 2024 |
| Optimus Engineering Limited | Professional & scientific | 2004 |
| OPTION17 Limited | Real estate | 2023 |
| Options East Midlands Limited | Construction | 2008 |
| Optipower Solutions Ltd | Administrative & support | 2023 |
| Optrax Limited | Information & communication | 2021 |
| OR Fitness Ltd | Other services | 2021 |
| Oracle SEO Ltd | Administrative & support | 2008 |
| Orange Bath Limited | Real estate | 2023 |
| Orange Birmingham II Limited | Real estate | 2021 |
| Orange Bricks Ltd | Real estate | 2015 |
| Orange Bristol Limited | Real estate | 2021 |
| Orange Cardiff Limited | Real estate | 2024 |
| Orange Durham Limited | Real estate | 2004 |
| Orange Edinburgh Limited | Real estate | 2025 |
| Orange EST8 Ltd | Real estate | 2013 |
| Orange Exeter Limited | Real estate | 2025 |
| Orange Hull Limited | Real estate | 2019 |
| Orange Lancaster Limited | Real estate | 2021 |