| Olivaritennis Ltd | Information & communication | 2025 |
| Olive & Barr Partnership LLP | | 2025 |
| Olive Fashion (UK) Co. Ltd | Wholesale & retail | 2025 |
| Olive Rose Consulting Limited | Professional & scientific | 2026 |
| Olivia TSE Art Ltd | Manufacturing | 2025 |
| Olla BAG Ltd | Wholesale & retail | 2025 |
| Ollies Random Acts of Kindness CIC | Wholesale & retail | 2024 |
| Olsi Alliance Ltd | Wholesale & retail | 2017 |
| Olyana Ltd | Wholesale & retail | 2025 |
| OM Housing Limited | Real estate | 2020 |
| OM Yachting Limited | Administrative & support | 2026 |
| OMA Enterprises Limited | Wholesale & retail | 2025 |
| Omari & Sons Ltd | Construction | 2024 |
| Omawo Limited | Professional & scientific | 2009 |
| Omega HEALTH2HOME Ltd | Public administration | 2025 |
| Omega Views Ltd | Real estate | 2024 |
| Omegacloud Trading Limited | Wholesale & retail | 2019 |
| Omera Eading Co., Ltd | Wholesale & retail | 2026 |
| Ometech Ltd | Professional & scientific | 2018 |
| Omicron Accountancy Limited | Professional & scientific | 2026 |
| OMK Business Limited | Wholesale & retail | 2025 |
| OMM Nikolova Ltd | Wholesale & retail | 2018 |
| Omnexis Tech Ltd | Information & communication | 2026 |
| Omni Galaxy Limited | Wholesale & retail | 2025 |
| Omnia Media LP | | 2020 |
| Omniserve BPO Ltd | Professional & scientific | 2025 |
| Omnispace Trading Ltd | Wholesale & retail | 2026 |
| Omotola Gold Limited | Wholesale & retail | 2025 |
| Omraplus Agency Ltd | Wholesale & retail | 2025 |
| ON Demand Security Ltd | Administrative & support | 2023 |
| ON One Level LLP | | 2020 |
| ON Point Moves Ltd | Transport & logistics | 2026 |
| ON Task Group Ltd | Hospitality | 2023 |
| ON the Road Solutions Ltd | Administrative & support | 2017 |
| On-Line Retails Shop Limited | Wholesale & retail | 2024 |
| Onbavira Workflow Setup Ltd | Information & communication | 2026 |
| Onca Global Ltd | Wholesale & retail | 2025 |
| Oncedrop Limited | Wholesale & retail | 2025 |
| One AI Studio Ltd | Information & communication | 2025 |
| One Beat Ltd | Information & communication | 2023 |
| One Call International Limited | Transport & logistics | 1998 |
| One Five One Group Limited | Wholesale & retail | 2026 |
| One Fragrance Ltd | Wholesale & retail | 2025 |
| One Friend Ltd | Information & communication | 2021 |
| One P Ltd | Information & communication | 2022 |
| One Percent LUX Limited | Wholesale & retail | 2026 |
| One Stop Family Support Services Limited | Health & social care | 2018 |
| One Sugar Please Ltd | Real estate | 2023 |
| Onelife Planning LLP | | 2010 |
| Onenazo Limited | Wholesale & retail | 2025 |
| Onenineonesix Ltd | Wholesale & retail | 2025 |
| Oneshome Communications Ltd | Wholesale & retail | 2024 |
| Onew International Co., Ltd | Wholesale & retail | 2023 |
| Onewhistle Ltd | Administrative & support | 2025 |
| Online Retailing Limited | Wholesale & retail | 2013 |
| Onlydogs Tech Ltd | Information & communication | 2025 |
| Onsite Mobile Ltd | Wholesale & retail | 2026 |
| Ontermann LP | | 2020 |
| Onthemove Mechanics Ltd | Wholesale & retail | 2025 |
| Onward Property Investments Limited | Real estate | 2026 |
| Onyx & Co Collective Ltd | Professional & scientific | 2025 |
| Oomooverse Ltd | Manufacturing | 2025 |
| Ooya Ltd | Administrative & support | 2024 |
| Opalcreative Limited | Wholesale & retail | 2025 |
| Opalwok Ltd | Wholesale & retail | 2025 |
| Opaurea Ltd | Wholesale & retail | 2024 |
| Open Arms Kington (Oak) CIC | Hospitality | 2021 |
| Open Doors Finance Limited | Professional & scientific | 2023 |
| Open Hearts Collective CIC | Professional & scientific | 2026 |
| Open Horizon Group Ltd | Wholesale & retail | 2026 |
| Openvideo Ltd | Information & communication | 2025 |
| Operation Rally CIC | Other services | 2025 |
| Operations Director Limited | Information & communication | 2024 |
| Operina Ltd | Information & communication | 2023 |
| Opexra Limited | Information & communication | 2025 |
| Ophir Guild Ltd | Wholesale & retail | 2025 |
| Ophthalmology Services Limited | Health & social care | 2026 |
| OPL Marketing Limited | Professional & scientific | 2021 |
| Optalex Optometric Services Limited | Health & social care | 2009 |
| Optical Design Limited | Professional & scientific | 1999 |
| Opticore One Stop Solution Ltd | Professional & scientific | 2025 |
| Optihire Solutions Ltd | Professional & scientific | 2025 |
| Optima Aesthetic Systems Ltd | Wholesale & retail | 2025 |
| Optima Occupational Health Consultancy Limited | Health & social care | 2009 |
| Optimal Trading Corporate Ltd | Wholesale & retail | 2023 |
| Optimalbuy Ltd | Wholesale & retail | 2024 |
| Optimasend Ltd | Information & communication | 2025 |
| Optimistical Limited | Administrative & support | 2024 |
| Optimum One Limited | Wholesale & retail | 2025 |
| Optisynth Group Ltd | Wholesale & retail | 2025 |
| Optivize Ltd | Professional & scientific | 2025 |
| Opto Village Freehold Limited | Real estate | 2025 |
| Opulence Car Group Limited | Wholesale & retail | 2025 |
| Opulent World Ltd | Information & communication | 2023 |
| Opusatelier Ltd | Wholesale & retail | 2025 |
| Oquo Filters Ltd | Wholesale & retail | 2025 |
| Orange Army Training Services Limited | Education | 2021 |
| Orange Monkey Ltd | Wholesale & retail | 2024 |
| Orange Valley Property Limited | Real estate | 2018 |
| ORB Trading Technologies Ltd | Information & communication | 2018 |