| Old Flatts Farm Pet Crematorium Limited | Other services | 2017 |
| Old Sheffield Road Business Park Limited | Real estate | 2007 |
| OLE Technical Ltd | Construction | 2023 |
| Olive Lane Management Company Limited | Real estate | 2023 |
| Oliver Holdings Ltd | Real estate | 2024 |
| Oliver Naughton Properties Ltd | Real estate | 2025 |
| Olivers Furniture Ltd | Wholesale & retail | 2021 |
| Olivia BLU Boutique Ltd | Other services | 2023 |
| Olivia Hair-Tutorial Services Ltd | Other services | 2023 |
| Oliwer Ltd | Transport & logistics | 2018 |
| Oliwer Properties Ltd | Real estate | 2023 |
| OLK Holdings Limited | Financial services | 2017 |
| OLK Limited | Wholesale & retail | 2014 |
| Ollax Ltd | Construction | 2025 |
| Ollen. Communication Design Studio Ltd | Professional & scientific | 2025 |
| Oluomo Care Services Ltd | Health & social care | 2025 |
| Olympus Fight Club Gym Ltd | Arts & recreation | 2026 |
| Olympus Pro Performance Coaching Ltd | Health & social care | 2022 |
| Olympus Training Ltd | Health & social care | 2022 |
| OM Health Harmonization Technologies Ltd | Information & communication | 2025 |
| OM Medics Limited | Professional & scientific | 2014 |
| OM Vaibhavee Ltd | Wholesale & retail | 2022 |
| OM Valet Ltd | Wholesale & retail | 2026 |
| Omar&Asmaa 4LIFE Limited | Wholesale & retail | 2018 |
| OME Mortgage Specialists Limited | Financial services | 2018 |
| Omega 2CLEANING Services Limited | Administrative & support | 2025 |
| Omega Brickwork Limited | Construction | 2008 |
| ON Advertising Ltd | Professional & scientific | 2007 |
| ON Road Removals Ltd | Transport & logistics | 2026 |
| ON Site Haulage Limited | Transport & logistics | 2005 |
| ON the Hoof (Food 2 Go) Ltd | Hospitality | 2023 |
| ON Time Delivery Services Ltd | Transport & logistics | 2017 |
| One 44 Dental Ltd | Health & social care | 2014 |
| One by One International Limited | Administrative & support | 2024 |
| One Choice Store Limited | Wholesale & retail | 2026 |
| One Click for YOU Ltd | Wholesale & retail | 2022 |
| One Flight Limited | Wholesale & retail | 2009 |
| One Health Medical Services (PVT) Limited | Real estate | 2020 |
| One MLS Engineering Ltd | Professional & scientific | 2025 |
| One Rigging Limited | Administrative & support | 2018 |
| One Rock Limited | Real estate | 2018 |
| One Stop Phone Solution Ltd | Other services | 2018 |
| Onecare Community Ltd | Health & social care | 2025 |
| Online Carpets Limited | Extraterritorial | 2014 |
| Online Clearance Outlet Ltd | Wholesale & retail | 2022 |
| Onset Care Limited | Health & social care | 2023 |
| Onwards NET TEN Ltd | Construction | 2025 |
| Onyx Essentials Ltd | Wholesale & retail | 2026 |
| Onyx Facility Management Limited | Administrative & support | 2025 |
| Onyx Keys Group Limited | Real estate | 2026 |
| Onyx Management & Consultancy Ltd | Professional & scientific | 2025 |
| Onyx Real Estate Ltd | Real estate | 2024 |
| Onyxcozmyx Tropical Invent Ltd | Wholesale & retail | 2020 |
| Open Consultancy Services Limited | Information & communication | 2016 |
| Open Waters Ltd | Wholesale & retail | 2020 |
| Openstep Limited | Professional & scientific | 2022 |
| Opex Fire Safety Limited | Public administration | 2019 |
| Opolex Ltd | Wholesale & retail | 2015 |
| OPT Painting Services Ltd | Construction | 2022 |
| Optare PLC | Manufacturing | 2008 |
| Optare UK Limited | Manufacturing | 2007 |
| Optic Nova Management Solutions Ltd | Information & communication | 2016 |
| Optical Technologies Ltd | Manufacturing | 2025 |
| Optimal Cladding Limited | Construction | 2020 |
| Optimise by Recruitment Holdings Ltd | Financial services | 2022 |
| Opulenta Global Solutions Limited | Wholesale & retail | 2014 |
| Opus Pilates Studio Ltd | Arts & recreation | 2025 |
| OR Services (U.K) Ltd | Health & social care | 2016 |
| Oracle Associates Ltd | Administrative & support | 2012 |
| Oracle Financial Advice Ltd | Financial services | 2015 |
| Orange Campers Hire Ltd | Administrative & support | 2022 |
| Orange Campers Ltd | Wholesale & retail | 2019 |
| Orange Investments Ltd | Administrative & support | 2020 |
| Orange Leaf Ltd | Professional & scientific | 2018 |
| ORB Fitness Training Ltd | Arts & recreation | 2018 |
| Orbis Nova CIC | Real estate | 2025 |
| Orca Learn Limited | Extraterritorial | 2023 |
| Orchard Court (Hope Valley) Management Company Limited | Real estate | 2020 |
| Orchard Court Management ( Thrybergh ) Limited | Households | 2020 |
| Orchard Court Property Ltd | Real estate | 2016 |
| Orchard Landscapes and Paving Limited | Administrative & support | 2023 |
| Orchids (Halo) Ltd | Health & social care | 2020 |
| Orcomm Ltd | Professional & scientific | 2026 |
| Oreo Services Ltd | Information & communication | 2023 |
| Oriental Chef Ltd | Hospitality | 2020 |
| Origin Aquatic Nutrition Ltd | Wholesale & retail | 2021 |
| Origin Fit OUT Limited | Construction | 2025 |
| Original Ohle Services Limited | Construction | 2014 |
| Orionum Ltd | Hospitality | 2025 |
| Orsfield Limited | Agriculture & fishing | 2025 |
| Orsova Trans Ltd | Transport & logistics | 2019 |
| Ortem Sails Ltd | Transport & logistics | 2015 |
| Orthopaedic Services South Yorkshire LLP | | 2025 |
| Osborne Properties Holdings Limited | Financial services | 2020 |
| Oshrock Limited | Professional & scientific | 2018 |
| Oshsquare Properties Limited | Real estate | 2023 |
| Ossfield Ltd | Real estate | 2024 |
| OTC Logistics & Auto Services Ltd | Other services | 2026 |
| Otherland Collective Limited | Manufacturing | 2024 |
| Otter Accommodation Limited | Extraterritorial | 2014 |