| Optima C Services Ltd | Real estate | 2022 |
| Optima Consulting & Advisory Ltd | Professional & scientific | 2023 |
| Optima HMO Properties Ltd | Real estate | 2022 |
| Optima Nursing & Care Service Ltd | Administrative & support | 2011 |
| Optima Overseas Recruitment Ltd | Administrative & support | 2017 |
| Optima RBS Managements Ltd | Real estate | 2022 |
| Optimal Healthcare Limited | Administrative & support | 2023 |
| Optimal Rehab Limited | Other services | 2024 |
| Optimise Utility Services Limited | Energy | 2020 |
| Optimum Beverages Limited | Manufacturing | 2020 |
| Optimum Corporation Limited | Information & communication | 1993 |
| Optisurge Limited | Professional & scientific | 2025 |
| Opulent Healthcare Limited | Health & social care | 2026 |
| Opus Management Installation Support Ltd | Construction | 2021 |
| Opusaura Limited | Wholesale & retail | 2024 |
| Opuslis Limited | Wholesale & retail | 2025 |
| Orangebox Supermarkets Limited | Wholesale & retail | 2019 |
| Orangebrook Associates Limited | Professional & scientific | 1999 |
| Orbis Groundworks Limited | Construction | 2018 |
| Orbis Plant Hire Limited | Construction | 2018 |
| Orbit Cable Services Ltd | Professional & scientific | 2021 |
| Orbit Direct Limited | Professional & scientific | 2008 |
| Orbit Essentials Ltd | Wholesale & retail | 2024 |
| Orbit Property Ltd | Real estate | 2025 |
| Orbital Facilities Management Limited | Professional & scientific | 2024 |
| Orbital Scientific Supplies Limited | Wholesale & retail | 2015 |
| Orca Engineering Ltd | Information & communication | 2015 |
| Orchard Community Trust | Education | 2018 |
| Orchard Electronics (Endon) Limited | Administrative & support | 2004 |
| Orchard House Property Developments Limited | Construction | 2015 |
| Order Employment Services Limited | Professional & scientific | 2023 |
| Order Management Group Limited | Extraterritorial | 2021 |
| Oremoxie Limited | Construction | 2026 |
| Oreo Scrap Metal & Rubbish Removals Ltd | Wholesale & retail | 2023 |
| Oreomed Ltd | Extraterritorial | 2020 |
| Orfffs Development Limited | Construction | 2025 |
| Organisciak Consulting Limited | Information & communication | 2016 |
| Oriental Carpets & Rugs Limited | Wholesale & retail | 2010 |
| Oriental Pearl Kidsgrove Ltd | Hospitality | 2024 |
| Oriental Pearl Ltd | Hospitality | 2017 |
| Origin NLS Limited | Transport & logistics | 2025 |
| Original Design UK Shop Ltd | Wholesale & retail | 2022 |
| Original Trade Limited | Wholesale & retail | 2019 |
| Orion Cctv Limited | Construction | 2021 |
| Orion Connect Limited | Information & communication | 2024 |
| Ormibox Limited | Wholesale & retail | 2023 |
| ORS (Luckington) Limited | Hospitality | 2001 |
| OSB Stoke Ltd | Wholesale & retail | 2025 |
| Oscar Training Services Ltd | Education | 2024 |
| Osprey Tyre Consultancy Limited | Professional & scientific | 2003 |
| Osprey Ventures Ltd | Manufacturing | 2024 |
| Osro Professional Ayurvedic Wellness Centre Limited | Health & social care | 2025 |
| Ostara Ecological Services Ltd | Professional & scientific | 2022 |
| Ostron Holdings Limited | Financial services | 2020 |
| OT Makes Sense Ltd. | Health & social care | 2017 |
| Otbot Digital Limited | Information & communication | 2026 |
| Otis Wolf Holdings Limited | Financial services | 2023 |
| OTT Building & Landscaping Ltd | Construction | 2024 |
| Oulton Farming Limited | Agriculture & fishing | 2025 |
| Oulton Surveyors and Auctioneers Limited | Professional & scientific | 2018 |
| OUR Altered Lives C.I.C. | Health & social care | 2024 |
| OUR IT Manager Ltd | Information & communication | 2011 |
| OUR Roots and Reasons Ltd | Agriculture & fishing | 2025 |
| OUR Secret Garden Staffs CIC | Agriculture & fishing | 2025 |
| OUT of Town Hairdressers Limited | Other services | 2014 |
| Outclassedu Ltd | Education | 2025 |
| Outlook Services Ltd | Construction | 2015 |
| Outrun Sports Company Ltd | Wholesale & retail | 2025 |
| Outside Edge Cheshire Ltd | Administrative & support | 2025 |
| Outside Inns Limited | Hospitality | 2005 |
| Outside the Square Limited | Professional & scientific | 2009 |
| Outwyz Digital Marketing Solutions Limited | Professional & scientific | 2025 |
| Oven Perfect Limited | Other services | 2016 |
| Overclockers UK Limited | Wholesale & retail | 2003 |
| Overseas Care Supplier Ltd | Administrative & support | 2021 |
| Overweight Ebony's Ltd | Wholesale & retail | 2026 |
| Ovevo Limited | Wholesale & retail | 2018 |
| Owen Construction (M/Cr) Limited | Construction | 2014 |
| Owens Fabrications Ltd | Manufacturing | 2017 |
| OWL Glistening Ltd | Wholesale & retail | 2026 |
| Owolabi Francis Ltd | Health & social care | 2024 |
| Oxerra Pigments UK Limited | Manufacturing | 1989 |
| Oxerra UK Limited | Wholesale & retail | 1989 |
| Oxford Reviews Ltd | Professional & scientific | 2025 |
| Oxley Denim Limited | Manufacturing | 2020 |
| Oyaji Ltd | Information & communication | 2011 |
| OYS Glow Limited | Wholesale & retail | 2023 |
| Oyster Lane Developments Ltd | Construction | 2022 |
| OZ Consultancy and Training Ltd | Education | 2023 |
| Oz&Zo Ltd | Wholesale & retail | 2022 |
| Ozaic Fireplace Factory Limited | Manufacturing | 2010 |
| Ozaic Online Limited | Administrative & support | 2017 |
| Ozden Global Companies Ltd | Wholesale & retail | 2025 |
| Ozmedi Ltd | Health & social care | 2020 |
| Ozombongo ZO Tukuise in UK CIC | Arts & recreation | 2026 |
| Ozone Recovery Limited | Wholesale & retail | 2016 |
| OZZ Sound and Lighting Ltd | Wholesale & retail | 2013 |
| Ozzytronics Technologies Limited | Manufacturing | 2024 |
| P & D Online Limited | Information & communication | 2011 |
| P & J Joinery Ltd | Construction | 2020 |