| Omega Alpha Equine Limited | Wholesale & retail | 2013 |
| OMNI360 Ltd | Water & waste | 2024 |
| Omnimedical Limited | Health & social care | 2003 |
| Omnipro Consulting Limited | Public administration | 2022 |
| Omrah Enterprises Ltd | Wholesale & retail | 2018 |
| ON ALL Floors QT Limited | Construction | 2017 |
| ON Catering Limited | Hospitality | 2024 |
| ON Foot Limited | Administrative & support | 2005 |
| On-Line Taxis Limited | Transport & logistics | 2008 |
| ON2 Media Ltd | Professional & scientific | 2011 |
| Onabowu Holdings Ltd | Arts & recreation | 2023 |
| Onboard Technology Holdings Limited | Wholesale & retail | 2020 |
| One + Point Limited | Administrative & support | 2024 |
| One Eight Zero (Bedhampton) Management Company Limited | Households | 2015 |
| One Life Bloodstock Limited | Agriculture & fishing | 2021 |
| One Life Group Limited | Financial services | 2019 |
| One Life Trust C.I.C. | Other services | 2021 |
| One Lockleaze (Bristol) Management Company Limited | Households | 2024 |
| One Point SIX Advertising Limited | Information & communication | 2015 |
| One Point Survey Equipment Limited | Administrative & support | 2013 |
| One Real Estate Group Limited | Professional & scientific | 2024 |
| One World Mindfulness Ltd | Health & social care | 2025 |
| One-Eight-One Services Limited | Administrative & support | 2024 |
| Onex Trading Group Holdings Ltd | Financial services | 2022 |
| Online City Cabs Ltd | Transport & logistics | 2026 |
| Onsite Medical Solutions Limited | Public administration | 2019 |
| Ontix Holdings LLP | | 2018 |
| Onut Transport Limited | Transport & logistics | 2019 |
| Onya Velo Ltd | Administrative & support | 2021 |
| Opal Wiltshire and Somerset Limited | Education | 2023 |
| Open Skies Engineering Ltd | Professional & scientific | 2023 |
| Operaluna Ltd | Arts & recreation | 1997 |
| Ophra Ltd | Administrative & support | 2025 |
| Ophthalmicorp Ltd | Other services | 2016 |
| Ophthotech Limited | Health & social care | 2014 |
| Opsec Projects Limited | Administrative & support | 2024 |
| Optica 2025 Limited | Manufacturing | 2025 |
| Optima Human Factors Ltd | Professional & scientific | 2020 |
| Optimal Planning Ltd | Financial services | 2006 |
| Optimus Water Services Ltd | Construction | 2015 |
| Opulence Bespoke Furniture Ltd | Manufacturing | 2023 |
| Opus HR Solutions Ltd | Administrative & support | 2023 |
| Orbis Insight Limited | Professional & scientific | 2015 |
| Orbital Eyewear Limited | Education | 2003 |
| Orbs Risk Ltd | Information & communication | 2022 |
| Orchard & Stone Ltd | Construction | 2025 |
| Orchard Bay Bakery Limited | Hospitality | 2016 |
| Orchard Chase Kirby Muxloe Management Company Limited | Households | 2010 |
| Orchard Farm (Cotswolds) Limited | Agriculture & fishing | 2017 |
| Orchard Gate (Kingston Bagpuize) Management Company Limited | Households | 2015 |
| Orchard Grove (Comeytrowe) Employment Area Management Company Limited | Households | 2025 |
| Orchard Grove (Comeytrowe) Management Company Limited | Households | 2021 |
| Orchard House (Salisbury) Management Company Limited | Real estate | 2016 |
| Orchard HR Limited | Professional & scientific | 2018 |
| Orchard Meadow Limited | Other services | 2025 |
| Ordino Global Ltd | Information & communication | 2025 |
| Ordnance ROW (Tisbury) Limited | Financial services | 2020 |
| Ordnance ROW Consulting Limited | Real estate | 2020 |
| Ordnance ROW Investments Ltd | Financial services | 2019 |
| Oreo Consulting Ltd | Information & communication | 2021 |
| Organaponics Limited | Professional & scientific | 2024 |
| Organaponics Nutrition Network C.I.C. | Professional & scientific | 2025 |
| Orlis Solutions Ltd | Financial services | 2024 |
| Orophilous Ltd | Extraterritorial | 2025 |
| Orsel Limited | Hospitality | 2020 |
| Orthogirl Limited | Health & social care | 2026 |
| Orthogonal Thinking Limited | Professional & scientific | 2025 |
| Orthopaedics Plastics & Spinal Specialists LLP | | 2012 |
| Orton Jones Limited | Manufacturing | 2015 |
| Orwell Studios (MP) Limited | Real estate | 2022 |
| Osaf Projects Limited | Construction | 2002 |
| Osborne & Collins Limited | Construction | 1994 |
| Osman Originals Limited | Other services | 2003 |
| Osman Racing Club Ltd | Arts & recreation | 2025 |
| Osman Racing Ltd | Arts & recreation | 2025 |
| Osmosis Production Services Limited | Administrative & support | 2020 |
| Osmund Engineering Limited | Wholesale & retail | 2016 |
| Ososilver Limited | Wholesale & retail | 2011 |
| Oswinseven Ltd | Professional & scientific | 1999 |
| OTE Precision Engineering (UK) Ltd | Manufacturing | 2012 |
| Otom Limited | Health & social care | 2024 |
| Otter & Hound PUB Company Limited | Hospitality | 2016 |
| Otter Vacuum Limited | Extraterritorial | 2014 |
| Otterbourne Retail Limited | Wholesale & retail | 2017 |
| OUR Community News Limited | Information & communication | 2019 |
| OUT of the Blue Studios Limited | Wholesale & retail | 2017 |
| Outside Context Ltd | Professional & scientific | 2020 |
| OV Corp Limited | Information & communication | 2014 |
| Oval Trustees Limited | Financial services | 1992 |
| Ovation Outdoor Ltd | Wholesale & retail | 2020 |
| Ovco Ltd | Construction | 2019 |
| Overseas Supply Services Limited | Professional & scientific | 1984 |
| Oxarix Ltd | Information & communication | 2013 |
| Oxby & Parke Recruitment Ltd | Administrative & support | 2005 |
| Oxeye Capital Management Limited | Financial services | 2000 |
| Oxford Spires Heating Ltd | Construction | 2000 |
| Ozfa Limited | Hospitality | 2025 |
| O’Driscoll & Company Limited | Professional & scientific | 2021 |
| P & E Homes Ltd | Construction | 2021 |
| P & F Gready Services Ltd | Financial services | 2018 |