| OGS Gardening Limited | Administrative & support | 2017 |
| OH I SAY UK Limited | Wholesale & retail | 2024 |
| OHG Resources Ltd | Other services | 2013 |
| OHM Picked Ltd | Wholesale & retail | 2019 |
| Ohms Group Limited | Construction | 2019 |
| OI Management Limited | Professional & scientific | 2022 |
| Oilyfingers Ltd | Wholesale & retail | 2025 |
| OJB and SON Limited | Construction | 2013 |
| OJD Trading Ltd | Financial services | 2026 |
| Ojii Ltd | Manufacturing | 2020 |
| Okaro Care Ltd | Health & social care | 2023 |
| Okaro Construction Ltd | Construction | 2022 |
| Okaro Property Ltd | Real estate | 2020 |
| Okcs Construction Supplies Ltd | Construction | 2024 |
| Oki-Doki Nails Limited | Other services | 2012 |
| Oksana Fruits Ltd | Wholesale & retail | 2023 |
| OLA and Sons Ltd | Public administration | 2018 |
| OLA Travel Ltd | Administrative & support | 2024 |
| Old Beams Commercial Limited | Real estate | 2024 |
| Old Beams Lettings Limited | Real estate | 2014 |
| Old House at Home Maidstone Ltd | Hospitality | 2024 |
| Old Kent Road Property Co. Limited | Households | 1988 |
| Old Mission Hall Residents Management Company Ltd | Households | 2025 |
| Old OIL & GAS Engines the Magazine Ltd | Information & communication | 2024 |
| Old Vicarage West Farleigh (Management Company) Limited(The) | Households | 1983 |
| Oldbury Stay Limited | Hospitality | 2026 |
| Olive Island Property Ltd | Real estate | 2022 |
| Olive's Nursery and Daycare Ltd | Health & social care | 2024 |
| Oliver and Alexander Ltd | Wholesale & retail | 2020 |
| Oliver Cameron Publishing Ltd | Other services | 2012 |
| Oliver K Properties Ltd | Real estate | 2025 |
| Oliver Montague Limited | Administrative & support | 2024 |
| Oliver Wellbeing Ltd | Health & social care | 2018 |
| Ollie Solman Limited | Information & communication | 2013 |
| Olly & RYE Electrical Ltd | Construction | 2023 |
| Olma Aesthetics Ltd | Wholesale & retail | 2026 |
| Olov Services Ltd | Construction | 2024 |
| Olson Partners LLP | | 2016 |
| Olufemi Ojogbede Limited | Health & social care | 2022 |
| Olympia Boxing CIC | Arts & recreation | 2017 |
| Oman Engineering Limited | Construction | 2021 |
| OMB Group Ltd | Professional & scientific | 2019 |
| OME Consulting Limited | Professional & scientific | 2018 |
| OMEGA24 Services Limited | Health & social care | 2018 |
| Omicron Technologies Ltd | Manufacturing | 2020 |
| Omiga Fire and Security Limited | Administrative & support | 2018 |
| OMK Services Limited | Information & communication | 2022 |
| Omni Synergy Ltd | Financial services | 2024 |
| OMO SHO Limited | Information & communication | 2022 |
| Ompatech Limited | Information & communication | 2026 |
| Omrooi Limited | Health & social care | 2024 |
| OMW Automotives Limited | Wholesale & retail | 2023 |
| Omweb Ltd | Information & communication | 2022 |
| ON Grade Earthworks and Stabilisation Ltd | Construction | 2023 |
| ON the Stick Limited | Hospitality | 2024 |
| ON Time Courier(Ro) Ltd | Transport & logistics | 2023 |
| On-Wood Products Limited | Manufacturing | 2009 |
| On-X Electrical Contractors Ltd | Construction | 2022 |
| On-X Group Limited | Extraterritorial | 2022 |
| Onaia Ltd | Professional & scientific | 2022 |
| Onark Group Limited | Professional & scientific | 2018 |
| Once Upon a Word Ltd | Arts & recreation | 2025 |
| One Customs Ltd | Administrative & support | 2025 |
| One Delivery Express Ltd | Transport & logistics | 2026 |
| One Green Bean London Limited | Professional & scientific | 2015 |
| One Hame Limited | Real estate | 2019 |
| One High Street Management Company Limited | Households | 2021 |
| One Infiniti Ltd | Real estate | 2021 |
| One Nine Homes Limited | Real estate | 2019 |
| One Polo (UK) Limited | Households | 2008 |
| One Polo Limited | Professional & scientific | 2017 |
| One Sundown Avenue Freehold Limited | Extraterritorial | 2005 |
| One Tree Properties Ltd | Households | 2025 |
| Oneil E-Commerce Ltd | Wholesale & retail | 2022 |
| Oneqube Ltd | Health & social care | 2022 |
| Onestopgym Ltd | Arts & recreation | 2023 |
| Onetwosecure Ltd | Other services | 2016 |
| Onic Electrical Ltd | Construction | 2011 |
| Onison Limited | Construction | 2003 |
| Online Media Post Ltd | Information & communication | 2019 |
| ONLINE95 Limited | Professional & scientific | 2011 |
| Onspec Ontic Limited | Other services | 1990 |
| Onyx BEE Limited | Information & communication | 2020 |
| Ooladeedah Limited | Wholesale & retail | 2014 |
| OPB Multitrade Ltd | Extraterritorial | 2023 |
| Open Doors Healthcare Services Ltd | Administrative & support | 2020 |
| Open ORG Group Ltd | Administrative & support | 2023 |
| Opera & Ballet Productions Limited | Arts & recreation | 2012 |
| Ophthalmology Clinical Services Ltd | Professional & scientific | 2020 |
| Opium Pulses Ltd | Information & communication | 2014 |
| Optic Chiasm Ltd | Health & social care | 2024 |
| Optima Court Management Company Coulsdon Limited | Households | 2009 |
| Optimal Interiors Limited | Construction | 2021 |
| Optimize Steel Services Ltd | Construction | 2022 |
| Optimorph Ltd | Professional & scientific | 2024 |
| Optiverse Limited | Administrative & support | 2025 |
| Optivolt Solutions Limited | Professional & scientific | 2025 |
| Optovue Ltd | Health & social care | 2016 |
| Opus Transport Limited | Transport & logistics | 2020 |
| Oram Consulting Ltd | Information & communication | 2026 |