| OI Specialist Services Group Ltd | Administrative & support | 2024 |
| OIL Flick Films NO.2 LLP | | 2004 |
| Ojay Associates Limited | Construction | 2003 |
| Okicx Limited | Construction | 2016 |
| Okpetoritse Ologban Limited | Health & social care | 2025 |
| Old Chapel Antiques Limited | Wholesale & retail | 2003 |
| Old Hall Energy Centre Limited | Energy | 2019 |
| Old Leigh Catering Limited | Hospitality | 2024 |
| Old Parsonage Court Residents Association Limited | Households | 1983 |
| Old Vacuumatic Ltd | Manufacturing | 1946 |
| Olgapimpino Ltd | Professional & scientific | 2025 |
| Oliveira World Ltd | Wholesale & retail | 2025 |
| Oliver's Italian Restaurant & Wine Bar Ltd | Hospitality | 2018 |
| Oliver's Lofts Ltd | Construction | 2021 |
| Olivex (UK) Limited | Information & communication | 2020 |
| Ollic Digital Ltd | Information & communication | 2025 |
| Olly-Kay's Limited | Health & social care | 2015 |
| Olph Ltd | Wholesale & retail | 2025 |
| Olympic Sign Services Limited | Professional & scientific | 1997 |
| Omaura Homes Ltd | Hospitality | 2024 |
| Omaura Medics Ltd | Health & social care | 2024 |
| Omega 1 Security Ltd | Administrative & support | 2022 |
| Omega Automotive Engineering Ltd | Information & communication | 2007 |
| Omega Personnel Limited | Administrative & support | 2020 |
| OMEGA3 Management Ltd | Real estate | 2025 |
| Omility Limited | Information & communication | 2024 |
| Omkan Limited | Information & communication | 2023 |
| Omnexa Ltd Ltd | Wholesale & retail | 2025 |
| Omni Live Ltd | Professional & scientific | 2023 |
| Omniinnovax Ltd | Information & communication | 2026 |
| Omthaver Consulting Ltd | Professional & scientific | 2024 |
| ON Brand International Limited | | 2003 |
| ON the Money Group Ltd | Administrative & support | 1992 |
| ON Track Estates Limited | Real estate | 2023 |
| Ona.Deliver Ltd | Transport & logistics | 2021 |
| One and Nine Apparel Creator Ltd | Wholesale & retail | 2022 |
| One Stop Marketing Agency (Osma) Ltd | Wholesale & retail | 2023 |
| One TEN Solutions Ltd | Information & communication | 2025 |
| ONE39 Solutions Ltd | Information & communication | 2017 |
| Onemall Ltd | Wholesale & retail | 2025 |
| Onetree Estates Limited | Health & social care | 1988 |
| Online Lubricants Limited | Wholesale & retail | 1994 |
| Online Projects Ltd | Wholesale & retail | 2026 |
| Online Will Write Ltd | Wholesale & retail | 2026 |
| Onroute Limited | Information & communication | 1987 |
| Ontraq Limited | Information & communication | 1998 |
| Onxfinit Ltd | Wholesale & retail | 2024 |
| Onyix Ltd | Construction | 2021 |
| Onyxx Games Ltd | Information & communication | 2025 |
| Ooomic Integrated Services Ltd | Professional & scientific | 2020 |
| Opale House Ltd | Real estate | 2026 |
| Opaleye Limited | Real estate | 2005 |
| Opecguard Limited | | 1985 |
| Opefabrik Ventures Ltd | Wholesale & retail | 2026 |
| Open FX Broker Ltd | Information & communication | 2026 |
| Openview Security Solutions Limited | Construction | 1997 |
| Openwaves Technology Ltd | Information & communication | 2024 |
| Opeyvariety Ltd | Wholesale & retail | 2012 |
| Opiol Servises Ltd | Professional & scientific | 2025 |
| Optimus Prime Health Care Limited | Health & social care | 2024 |
| Opulent Luxury Limited | Wholesale & retail | 2025 |
| ORA Tourism Services Ltd | Wholesale & retail | 2026 |
| Orbit Associates UK Limited | Extraterritorial | 2005 |
| Orbit Distribution Limited | Other services | 2000 |
| Orbital Essex Fastners Limited | Wholesale & retail | 2006 |
| Orbital Welding (London) Limited | | 1975 |
| Order Efficiency Limited | Professional & scientific | 2005 |
| Ordercad Ltd | Extraterritorial | 2022 |
| Organic Beauty Limited | Extraterritorial | 2005 |
| Organisation of Business Professionals Ltd | Education | 2012 |
| Orient Dynamics Consulting Ltd | Information & communication | 2018 |
| Orient Enterprises Limited | Other services | 2019 |
| Origin Wear Ltd | Wholesale & retail | 2023 |
| Orion Sales Ltd | Wholesale & retail | 2026 |
| Orlisse Ltd | Wholesale & retail | 2026 |
| Orsett Horticultural and Agricultural Society | Other services | 1992 |
| Orwell & Allen Limited | | 1978 |
| Osborne Motor Bodies Limited | Other services | 2005 |
| Oscar Breakfast House Ltd | Hospitality | 2024 |
| Osinachi and Chiamaka Ltd | Health & social care | 2024 |
| Osiyemi Syndicated Investments Ltd | Financial services | 2026 |
| Oslo MED Solutions Ltd | Health & social care | 2024 |
| Osman Discount Hub Ltd | Wholesale & retail | 2026 |
| Other Vision Solutions Ltd | Administrative & support | 2021 |
| Otss Limited | Construction | 2005 |
| Ottoman Wine Company Limited | Real estate | 1979 |
| Ottomos Ltd | Information & communication | 2024 |
| Oudigi Ltd | Information & communication | 2026 |
| Our-Scent Ltd | Wholesale & retail | 2016 |
| Outbreak Leisure Limited | Real estate | 2003 |
| Outlook Outdoor Media Ltd | Professional & scientific | 2006 |
| Oval Wall Ltd | Real estate | 2021 |
| Overlander Vehicles Limited | Wholesale & retail | 2018 |
| Overton Construction & Engineering Limited | Construction | 2003 |
| Ovile Ltd | Wholesale & retail | 2026 |
| Own Shop Ltd | Wholesale & retail | 2025 |
| OWT Group Ltd | Administrative & support | 2021 |
| OXA Capital Limited | Financial services | 2025 |
| Oxney Forge Ltd | Agriculture & fishing | 2008 |
| Oxonburg Ltd | Manufacturing | 2026 |