| One Canon Limited | Financial services | 2017 |
| One Queens Place Ltd | Households | 2013 |
| One Source Solutions Ltd. | Wholesale & retail | 2018 |
| One Stop Mobile Repair Centre Limited | Wholesale & retail | 2017 |
| One Stop Stores ( Fairford Leys) Ltd | Wholesale & retail | 2021 |
| One Vision Technologies Ltd | Wholesale & retail | 2024 |
| One White Chair Limited | Professional & scientific | 2010 |
| One World Closeouts Ltd | Other services | 2006 |
| Onehive at a Time CIC | Education | 2025 |
| Onehive Group Ltd | Information & communication | 2024 |
| Onehub Properties Ltd | Real estate | 2025 |
| Onehundredluxury Ltd | Wholesale & retail | 2020 |
| Onelex Limited | Administrative & support | 2007 |
| Onetent Limited | Real estate | 2025 |
| Online Bargain Limited | Wholesale & retail | 2020 |
| Online Wellness Limited | Wholesale & retail | 2019 |
| Only Local Express Limited | Wholesale & retail | 2023 |
| Onlyme Limited | Information & communication | 2025 |
| Ontheup Ltd | Wholesale & retail | 2026 |
| Ontime Airports Limited | Transport & logistics | 2014 |
| ONTIME4YOU Haulage Ltd | Transport & logistics | 2022 |
| Onwards Coaching Ltd | Professional & scientific | 2023 |
| Onyx Construction & Properties Ltd | Real estate | 2013 |
| Onyx One Bushey Limited | Real estate | 2025 |
| Onyx Properties (UK) Limited | Real estate | 2011 |
| OO24 Watches and Jewellery Ltd | Wholesale & retail | 2018 |
| OOO Interiors UK Ltd | Construction | 2015 |
| Opal Clinic Limited | Health & social care | 2013 |
| Opal Clinic London Ltd | Health & social care | 2019 |
| Opal Court N3 Freehold Ltd | Households | 2018 |
| OPC2. Limited | Real estate | 2025 |
| Open Book Open Soul Ltd | Education | 2022 |
| Openstar Limited | Information & communication | 2005 |
| Opera Realty Ltd | Real estate | 2013 |
| OPM (Edgware) Limited | Other services | 2010 |
| Opnex Limited | Professional & scientific | 2012 |
| Opogo Education Limited | Information & communication | 2026 |
| Opogo Limited | Information & communication | 2017 |
| Opogo Services Limited | Information & communication | 2026 |
| Opsys Insights Ltd | Administrative & support | 2025 |
| Optima Glazing Ltd | Construction | 2026 |
| Optimal Impact Ltd | Professional & scientific | 2019 |
| Optimedia Group Ltd | Manufacturing | 2016 |
| Optimum Legal Services Limited | Professional & scientific | 2020 |
| Optimusic I-Tech Limited | Manufacturing | 2005 |
| Options Group (UK) Limited | Administrative & support | 1999 |
| Optonica Lighting UK Ltd | Wholesale & retail | 2025 |
| Opulent Honors Ltd | Wholesale & retail | 2025 |
| Oqtet Limited | Information & communication | 2013 |
| Oralution Limited | Information & communication | 2017 |
| Orange Tree Property Developments Limited | Real estate | 2012 |
| Orange Trees Sicily Ltd | Agriculture & fishing | 2017 |
| Orberin Ltd | Professional & scientific | 2025 |
| Orbistrek Limited | Information & communication | 2003 |
| Orbit Autos Limited | Wholesale & retail | 2021 |
| Orbit Grill Limited | Hospitality | 2023 |
| Orbit Partnership Limited | Real estate | 2024 |
| Orchard Pharm Limited | Health & social care | 2002 |
| Orchard Place Limited | Households | 2025 |
| Orchard Real Estate (GBR) Limited | Real estate | 2023 |
| Orchid Lakes Ltd | Agriculture & fishing | 2024 |
| Orderstone Developments Limited | Real estate | 1999 |
| Organic Sury Limited | Wholesale & retail | 2020 |
| Orgreen Ltd | Wholesale & retail | 2010 |
| Oriens Astrum Limited | Information & communication | 2015 |
| Orient&Persian Books Ltd | Wholesale & retail | 2022 |
| Oriental & Fusion Food Ltd | Real estate | 2014 |
| Oriental 29 Ltd | Hospitality | 2024 |
| Oriental Foods Limited | Manufacturing | 1995 |
| Oriental House Limited | Hospitality | 2026 |
| Oriental Mart 365 Limited | Wholesale & retail | 2024 |
| Oriental Ventures Holding Ltd | Manufacturing | 2020 |
| Origin Design and Engineering Services Ltd | Real estate | 2011 |
| Original Dubai Shawarma Edgware Road Ltd | Hospitality | 2025 |
| Original Dubai Shawarma Greenford Ltd | Hospitality | 2025 |
| Original Dubai Shawarma Wembley Ltd | Hospitality | 2025 |
| Originals Academy Ltd | Other services | 2024 |
| Orizzont Ltd | Construction | 2025 |
| Orlando Hotel Limited | Real estate | 2021 |
| Orroja Limited | Professional & scientific | 2021 |
| Orthodontic Laboratory Ltd | Manufacturing | 2023 |
| Orthopaedic Imaging Limited | Health & social care | 2019 |
| Orvoir Ltd | Information & communication | 2024 |
| Osborn Ergonomics Limited | Professional & scientific | 2014 |
| OSP Tech Ltd | Information & communication | 2014 |
| Osteopathic Works Limited | Health & social care | 2011 |
| OSTEOPATHY4U Limited | Health & social care | 2008 |
| Osteoporosis Assessment Service Limited | Health & social care | 1997 |
| Otriana Ltd | Professional & scientific | 2020 |
| Otsumami Limited | Hospitality | 2017 |
| Otter Psychological Services Ltd | Education | 2022 |
| OUD & Aroma Ltd | Wholesale & retail | 2023 |
| OUR Youth Matter Ltd | Health & social care | 2023 |
| Ousheng Limited | Wholesale & retail | 2019 |
| Outersection Media Limited | Professional & scientific | 2018 |
| Outfitters Limited | Wholesale & retail | 2023 |
| Outstanding Cleaning and Maintenance Limited | Construction | 2024 |
| Ouvah Highfields Limited | Arts & recreation | 1967 |
| Ovania Limited | Real estate | 2004 |
| Ovenpride Ltd | Extraterritorial | 2021 |