| Obrimpong Healthcare Services Limited | Health & social care | 2022 |
| OC Club Catering Ltd | Hospitality | 2023 |
| OC401488 LLP | | 2015 |
| Ocapx Ltd | Financial services | 2025 |
| Ocean Harmony Ship Management Co., Ltd | Transport & logistics | 2024 |
| OCEAN7 Engineering Ltd | Professional & scientific | 2022 |
| Oceanic International Shipping UK Ltd | Administrative & support | 2018 |
| Ockram Limited | Manufacturing | 2005 |
| OCR Homes Ltd | Real estate | 2024 |
| Oddkid Films Limited | Information & communication | 2007 |
| Odva Ltd | Wholesale & retail | 2021 |
| Odypharm Limited | Administrative & support | 2005 |
| Odyssey Property Company | Real estate | 2000 |
| Oenophile Limited | Professional & scientific | 2018 |
| OFF Camber Solutions Limited | Hospitality | 2023 |
| OFF the Record Media Ltd | Professional & scientific | 2026 |
| Offer Consulting Limited | Professional & scientific | 2019 |
| Offer Estate Holdings Limited | Real estate | 2014 |
| Offer Properties Limited | Real estate | 1986 |
| Offer Real Limited | Real estate | 2021 |
| Offgrid Media Ltd | Information & communication | 2025 |
| Office & Dragons Ltd. | Information & communication | 2019 |
| Office Interiors D&B UK LLP | | 2016 |
| Official Design Services Ltd | Professional & scientific | 2019 |
| Offshore Medics Ltd | Health & social care | 2023 |
| Offsite Bases Limited | Construction | 2022 |
| OG Design & Build Ltd | Construction | 2025 |
| Ogwr Care Ltd | Health & social care | 2018 |
| OHA KT2 Limited | Hospitality | 2026 |
| Ohio Energy Limited | Energy | 2020 |
| Ojsdigital Limited | Information & communication | 2026 |
| OJY Limited | Professional & scientific | 2015 |
| Okati Limited | Information & communication | 2024 |
| Okay Travel Ltd | Hospitality | 2023 |
| Oklo South Mali Limited | Financial services | 2020 |
| Old Barn Autos Limited | Wholesale & retail | 2025 |
| Old Freemen's Rugby Football Club Limited | Arts & recreation | 2023 |
| Old Tollgate (West Sussex) Restaurant Limited | Hospitality | 1974 |
| Olef Ltd | Real estate | 2025 |
| Oliva AI Limited | Information & communication | 2026 |
| Oliver Oliver Ltd | Information & communication | 2014 |
| OMB Partnership Limited | Administrative & support | 2006 |
| Omega Lodge Limited | Households | 2005 |
| Omega One Consulting Ltd | Real estate | 2011 |
| Omenaibe Services Limited | Administrative & support | 2022 |
| Omnichannel Consulting Limited | Information & communication | 2013 |
| Omniya Group Limited | Real estate | 2025 |
| ON Thames Solutions Ltd | Professional & scientific | 2018 |
| One Air Ltd | Transport & logistics | 2021 |
| One HAT Press Limited | Information & communication | 2021 |
| One Hundred Knightsbridge Estates Ltd | Extraterritorial | 2014 |
| One Nine Seven Limited | Other services | 2000 |
| One Norbiton | Other services | 2012 |
| One Point Hub Ltd | Real estate | 2020 |
| One Tribe Global Ltd | Information & communication | 2019 |
| One Two One Global Ltd | Wholesale & retail | 2023 |
| One Way Fit App Limited | Information & communication | 2025 |
| ONE1000 Limited | Professional & scientific | 2002 |
| Onecloud Networks Limited | Information & communication | 2016 |
| Onemore Properties Limited | Real estate | 2020 |
| Onenineeight Richmond Limited | Extraterritorial | 2009 |
| Onesleep Ltd | Information & communication | 2025 |
| Onigiri Kome Ltd | Hospitality | 2024 |
| Onix Exploration UK Limited | Financial services | 2026 |
| Onlydent Ltd | Wholesale & retail | 2013 |
| Onroute Logistics Holdings Ltd | Transport & logistics | 2021 |
| Ontalia Ltd | Financial services | 2020 |
| Oodles Dogs Ltd | Manufacturing | 2024 |
| Opal Analysis Ltd | Wholesale & retail | 2017 |
| Opal STL Limited | Financial services | 2013 |
| Open Doors Education Ltd | Education | 2025 |
| Open EYE Post Ltd | Information & communication | 2017 |
| Ophp Ltd | Health & social care | 2023 |
| Opix Films Limited | Information & communication | 2009 |
| OPR Limited | Administrative & support | 2008 |
| Optiheal Healthcare Ltd | Wholesale & retail | 2023 |
| Optima Technology Limited | Information & communication | 2023 |
| Optimising Opportunities Ltd | Real estate | 2020 |
| Optimiza Ltd | Professional & scientific | 2024 |
| Optionsbiz Limited | Manufacturing | 2020 |
| Orafin Limited | Real estate | 1995 |
| Orange Antler Consulting Ltd | Professional & scientific | 2020 |
| Orange Gorilla Ltd | Administrative & support | 2018 |
| Orato Consulting Limited | Administrative & support | 2011 |
| Orchard Homes Private Limited | Health & social care | 2016 |
| Orchard Law Consulting Ltd | Professional & scientific | 2013 |
| Orchard Rise Management Limited | Households | 2003 |
| Orchard Riverside Development Ltd | Construction | 2022 |
| Orinoco Professional Limited | Professional & scientific | 2007 |
| Ormond House Media Limited | Information & communication | 2021 |
| Ormonde Corporate Advisory Limited | Professional & scientific | 2019 |
| Orpheus Project One Ltd | Administrative & support | 2017 |
| Orso Films Limited | Information & communication | 2015 |
| Orteq Sports Medicine Limited | Manufacturing | 2017 |
| Orto BIO Limited | Professional & scientific | 2024 |
| Osagie Omenai Limited | Construction | 2011 |
| Osborne Court (Ealing) Management Company Limited | Households | 1967 |
| Osborne Court Residents Company (Surbiton) Limited | Households | 1964 |
| Oscar Education Group Ltd | Administrative & support | 2026 |
| Oscar Education Limited | Transport & logistics | 2019 |