| Oliver MAY Bespoke Ltd | Manufacturing | 2021 |
| Oliver ST James Capital Partners Ltd | Construction | 2016 |
| Oliver ST James Limited | Real estate | 2008 |
| Oliver ST James New Homes Ltd | Construction | 2020 |
| Olivia Online Retail Limited | Wholesale & retail | 2026 |
| Ollipop & Co. Soaps Ltd | Manufacturing | 2024 |
| Olto Construction Ltd | Construction | 2021 |
| Olya Coffee Ltd | Hospitality | 2021 |
| OM Interactive Ltd | Administrative & support | 1997 |
| OM QP Services Limited | Professional & scientific | 2021 |
| Oman Tech Limited | Wholesale & retail | 2024 |
| Omatewe Limited | Health & social care | 2013 |
| Omni-Via Ltd | Manufacturing | 2024 |
| Omniquity Limited | Wholesale & retail | 2005 |
| Omniteam Limited | Administrative & support | 2011 |
| OMV Engineering Services Limited | Manufacturing | 2014 |
| OMV Holdings Limited | Financial services | 2024 |
| OMV SPV Limited | Real estate | 2025 |
| ON Point Plumbing and Heating Services Limited | Construction | 2024 |
| ON the Call Sheet Ltd | Professional & scientific | 2025 |
| ON Time Filming Ltd | Professional & scientific | 2019 |
| One Direction Accounting Services Ltd | Professional & scientific | 2025 |
| One for ALL Limited UK Limited | Information & communication | 2021 |
| One Grand Vision Construction Ltd | Construction | 2025 |
| One Istanbul Kebab Ltd | Hospitality | 2017 |
| One Stop Auto Hub Ltd | Wholesale & retail | 2018 |
| One Stop Dental Practice Limited | Health & social care | 2016 |
| One Stop Doctors Ltd | Health & social care | 2015 |
| One Stop International Business Formation Limited | Administrative & support | 2014 |
| One Stop Private Doctors Limited | Health & social care | 2014 |
| Onelink Fxconnect Limited | Information & communication | 2017 |
| Onelove Wellness Center Ltd | Health & social care | 2023 |
| ONEPOINT2 Leisure Limited | Real estate | 2021 |
| Onkar London Ltd | Wholesale & retail | 2025 |
| Online Bgstore Limited | Wholesale & retail | 2019 |
| Onlineatoz Ltd | Wholesale & retail | 2025 |
| Only Streetwise Ltd | Real estate | 2025 |
| Onpoint Accounting - Hemel Hempstead Limited | Professional & scientific | 2018 |
| Onpoint Accounting - Kent Limited | Professional & scientific | 2019 |
| Onskil Ltd | Information & communication | 2021 |
| Ontex Retail UK Limited | Wholesale & retail | 1982 |
| Ooshhstudio Ltd | Professional & scientific | 2019 |
| Opal Executive Cars Ltd | Transport & logistics | 2022 |
| Open Counselling Limited | Professional & scientific | 2012 |
| Open Estates Ltd | Real estate | 2013 |
| Open Hairdressing Academy Ltd | Other services | 2021 |
| Open Hairdressing Hemel Hempstead Limited | Other services | 2018 |
| OPK Limited | Arts & recreation | 2015 |
| Optic Security Services Ltd | Administrative & support | 2022 |
| Optimal Cleaning Ltd | Administrative & support | 2018 |
| Optimise Network Ltd | Transport & logistics | 2019 |
| Optimodo Limited | Construction | 2009 |
| Optimodo Marketing Limited | Professional & scientific | 2019 |
| Optipak Limited | Manufacturing | 1984 |
| Optoma Europe Ltd | Wholesale & retail | 1997 |
| Optoma Holding Limited | Wholesale & retail | 2021 |
| Optovate Limited | Professional & scientific | 2008 |
| Opts Holdings Ltd | Real estate | 2023 |
| Opulence VIP Vehicle Services Ltd | Administrative & support | 2024 |
| Opus Construct Ltd | Construction | 2012 |
| Opus Luxury Management Limited | Professional & scientific | 2015 |
| OR7 Properties Ltd | Real estate | 2018 |
| Oradent Leverstock Green Ltd | Health & social care | 2022 |
| Orange Tree Properties Limited | Real estate | 2017 |
| Orange Tree Solutions Limited | Construction | 2025 |
| Orca Business Solutions Ltd | Information & communication | 2010 |
| Orchard 1 Management Company Limited | Households | 2013 |
| Orchard Brooks (Williton) Management Company Limited | Households | 2021 |
| Orchard Court (Bovingdon) Limited | Households | 1997 |
| Orchard Court Sea Road Management Company Limited | Households | 2005 |
| Orchard Gardens (Melbourn) Limited | Households | 2021 |
| Orhan Mart Ltd | Wholesale & retail | 2026 |
| Orifarm Healthcare UK Ltd | Administrative & support | 2013 |
| Orifarm UK Ltd | Administrative & support | 2016 |
| Original Handmade Kitchens Ltd | Manufacturing | 2026 |
| Original Leak Detective Ltd | Professional & scientific | 2024 |
| Original Socialjems Limited | Professional & scientific | 2020 |
| Orion Building Engineering Services Limited | Construction | 1998 |
| Orion Heating Services Limited | Extraterritorial | 2012 |
| Orion Proxima Ltd | Information & communication | 2026 |
| ORL Trading Ltd | Wholesale & retail | 2022 |
| Orlando & Sons Holdings Limited | Real estate | 2024 |
| Orlando Refrigeration & Air Conditioning Limited | Manufacturing | 2015 |
| Orus Technical Services Ltd | Construction | 2025 |
| Osache Ltd | Education | 2022 |
| Oscar Friedheim Limited | Manufacturing | 1999 |
| OSD Healthcare Endoscopy Centre Limited | Health & social care | 2023 |
| Osgen Ltd. | Real estate | 2015 |
| Osgen Pharmaceuticals Limited | Manufacturing | 2016 |
| Osito Limited | Professional & scientific | 2018 |
| Oslabs Ltd | Professional & scientific | 2026 |
| OSM Property Investment Ltd | Real estate | 2025 |
| OSO Travtech Ltd | Information & communication | 2026 |
| Osprey Close Management Company Limited | Real estate | 2013 |
| Ossett Recruitment Ltd | Administrative & support | 2025 |
| OTE Park (Burgess Hill) Management Limited | Households | 2012 |
| OTO IT Consulting Limited | Information & communication | 2023 |
| Otranto Capital LLP | | 2013 |
| Ottimo Property Services Limited | Other services | 2009 |
| Otto&Chris Gardening Ltd | Transport & logistics | 2024 |