| Onward Information Services Limited | Information & communication | 2018 |
| Onyx Assets Group Ltd | Real estate | 2017 |
| Onyx Aureum Limited | Financial services | 2025 |
| Onyx HQ Ltd | Education | 2024 |
| Onyx Lagree and Co Ltd | Arts & recreation | 2025 |
| OO LA LA Ltd | Construction | 2019 |
| Ooblanc Music Ltd | Information & communication | 2022 |
| Op-Tix Consultancy Ltd | Information & communication | 2024 |
| OP2 Limited | Professional & scientific | 2018 |
| Opal AM Ltd | Other services | 2025 |
| Opal Development Projects Limited | Professional & scientific | 2013 |
| Opal Property Services Ltd | Real estate | 2024 |
| Opcom Rail Ltd | Information & communication | 2025 |
| Open Kitchen Delights Limited | Hospitality | 2024 |
| Open Minds Counselling Service Ltd | Health & social care | 2008 |
| Open World Education Limited | Professional & scientific | 2019 |
| Opencast C.I.C. | Education | 2002 |
| Opendoor Homes Ltd | Real estate | 2025 |
| Opening Credits Limited | Professional & scientific | 2022 |
| Opentech Group Ltd | Information & communication | 2019 |
| Opera Spedition Ltd | Transport & logistics | 2020 |
| Opper & Webb Limited | Wholesale & retail | 2013 |
| Opportune Holdings Ltd | Real estate | 2024 |
| Optanex Holding Ltd | Professional & scientific | 2017 |
| Opteco Ltd | Information & communication | 2023 |
| Opti-Pharma (UK) Limited | Manufacturing | 2014 |
| Optical Data Comms Ltd | Information & communication | 2020 |
| Optima Media (UK) Limited | Wholesale & retail | 2011 |
| Optima House Limited | Financial services | 1979 |
| Optima International Limited | Manufacturing | 1961 |
| Optima Kitchens Ltd | Manufacturing | 2024 |
| Optima Renewable Energy Ltd | Construction | 2025 |
| Optimal Nutrition Ltd | Other services | 2017 |
| Optimal Salts Ltd | Manufacturing | 2024 |
| Optimal Spray Ltd | Construction | 2026 |
| Optime Support Limited | Administrative & support | 2012 |
| Optimum Clean Professional Cleaning Service Ltd | Administrative & support | 2019 |
| Optimum Commercial Services Limited | Construction | 2014 |
| Optimum Developments & Property Services Ltd | Real estate | 2014 |
| Optimum Fire and Security Limited | Administrative & support | 2020 |
| Optimum Home Cleaning Ltd | Administrative & support | 2023 |
| Optimum Joinery Doncaster Limited | Construction | 2019 |
| Optimum Pain Management Ltd | Health & social care | 2017 |
| Optimum Site Specialists Ltd | Construction | 2014 |
| Optimum Systems Limited | Manufacturing | 1982 |
| Optimus Advies LLP | | 2014 |
| Optimus Digital Consulting Ltd | Information & communication | 2021 |
| Optum Global Ltd | Real estate | 2009 |
| Optwell Ltd | Health & social care | 2024 |
| Opulent Build Co. Ltd | Construction | 2025 |
| Opus Medical Limited | Health & social care | 2011 |
| Oracle Enterprises Ltd | Real estate | 2026 |
| Oracle Holdings Ltd | Financial services | 2025 |
| Oracle Insights Ltd | Education | 2026 |
| Oracle Rendering & Plastering Solutions (Orps) Limited | Construction | 2015 |
| Oracle Training Consultants Limited | Professional & scientific | 1999 |
| Orange & Pink Ltd | Professional & scientific | 2025 |
| Orange Strike Transportation Ltd Limited | Transport & logistics | 2026 |
| Orange Tree Electrical Services Ltd | Construction | 2013 |
| Orathai-Philip Design Ltd | Professional & scientific | 2022 |
| ORB Building Systems Limited | Construction | 2019 |
| ORB Connect Ltd | Information & communication | 2021 |
| Orba Engineering Boilers Ltd | Other services | 2018 |
| Orbit Security Ltd | Administrative & support | 2020 |
| Orbital Instruments Limited | Professional & scientific | 2008 |
| Orbitanexor Ltd | Wholesale & retail | 2026 |
| Orbitra Solutions Ltd | Extraterritorial | 2026 |
| Orchard Equine Transport Ltd | Transport & logistics | 2024 |
| Orchard Grove Management Company (Blyth) Limited | Households | 2021 |
| Orchard Workshops Ltd | Construction | 2014 |
| Orchardgate Foods Ltd | Extraterritorial | 2026 |
| Orchids Care Limited | Health & social care | 2009 |
| Orchids Respite Services Ltd | Public administration | 2019 |
| ORE Market Ltd | Wholesale & retail | 2025 |
| Ores Display Limited | Other services | 2015 |
| Organic Property Holdings Ltd | Administrative & support | 2021 |
| Organic Thought Ltd | Extraterritorial | 2025 |
| Organic Ventures Limited | Administrative & support | 2016 |
| Organised Print Limited | Manufacturing | 2013 |
| Oriental Chinese Meals Limited | Hospitality | 2017 |
| Origin Payroll Limited | Administrative & support | 2021 |
| Origin Tattoo Ltd | Arts & recreation | 2018 |
| Original Doner Ltd | Hospitality | 2024 |
| Original Heritage UK Ltd | Administrative & support | 2017 |
| Orion Core Logistics Ltd | Wholesale & retail | 2025 |
| Orku Limited | Wholesale & retail | 2019 |
| ORL Surveying Ltd | Professional & scientific | 2015 |
| Ormond Collective Limited | Extraterritorial | 2026 |
| Ormonde Homes Limited | Construction | 2015 |
| Orthowise Drsolution Ltd | Manufacturing | 2009 |
| Ortus Secured Finance III Limited | Financial services | 2014 |
| Orus Electrification Services Ltd | Construction | 2023 |
| Orvantis Digital Ltd | Extraterritorial | 2026 |
| Orvanyx Trading Group Ltd | Extraterritorial | 2026 |
| Orynex Holdings Ltd | Extraterritorial | 2026 |
| OS & Sons Vehicle Movement Ltd | Transport & logistics | 2026 |
| OS Education Ltd | Education | 2021 |
| OS Homes & Property Ltd | Real estate | 2025 |
| Os&Do Limited | Transport & logistics | 2023 |
| Oscar Gregson TM Ltd | Public administration | 2025 |