| Oakhurst Property Services Ltd | Construction | 2005 |
| Oakleaf Builders of Sevenoaks Limited | Construction | 2014 |
| Oakleigh Carpentry & Construction Ltd | Construction | 2022 |
| Oakleigh Exchange Partners Ltd | Financial services | 2024 |
| Oakley Construction (Kent) Limited | Construction | 2014 |
| Oakley Firearms Ltd | Wholesale & retail | 2025 |
| Oaklow Properties Limited | Real estate | 2014 |
| Oakmead Opticians Limited | Wholesale & retail | 2005 |
| Oakmont Services Ltd | Construction | 2025 |
| Oaks Advisory Limited | Administrative & support | 2018 |
| Oaks Asset Management LLP | | 2011 |
| Oaks C&C Carpentry & Construction Ltd | Construction | 2006 |
| Oaks Cleaning Services Ltd | Administrative & support | 2024 |
| Oaks Construction Enterprises Ltd | Financial services | 2018 |
| Oaks Electrical Supplies Ltd | Construction | 2017 |
| Oaks Home Improvements Ltd | Construction | 2024 |
| Oaks Plant Hire Limited | Administrative & support | 1970 |
| Oaks Retail Ltd | Wholesale & retail | 2023 |
| Oaks Tax & Accounts Ltd | Professional & scientific | 2018 |
| Oaks Taxis Limited | Transport & logistics | 2013 |
| Oakseven Properties Ltd | Real estate | 2013 |
| Oaksmore Care Facilites Limited Partnership | | 2016 |
| Oaksmore Heritage Property Limited Partnership | | 2016 |
| Oakstead Investments Ltd | Real estate | 2014 |
| Oakwood Beauty Limited | Other services | 2014 |
| Oakya Accountancy Limited | Professional & scientific | 2020 |
| Oasis Properties Limited | Real estate | 1983 |
| Oast to Coast Limited | Hospitality | 2017 |
| Oasthouse Marketing Design and Print Limited | Manufacturing | 2014 |
| Oberlin Ltd | Professional & scientific | 2022 |
| OBI Winter Beekeeping Limited | Agriculture & fishing | 2025 |
| Obsidian Capital Holdings Limited | Professional & scientific | 2024 |
| Ocane Ltd | Professional & scientific | 2024 |
| Occult Limited | Information & communication | 2022 |
| Occupational Services Solutions Limited | Health & social care | 2015 |
| Octofad Ltd | Professional & scientific | 2020 |
| Oddball Events Limited | Hospitality | 2016 |
| OFF Market Realty UK Ltd | Extraterritorial | 2008 |
| Offgrid.Finance Limited | Professional & scientific | 2020 |
| Officers View Management Company Ltd | Households | 2020 |
| Offset Architects Limited | Administrative & support | 2009 |
| Ogilvie Advisers Ltd | Professional & scientific | 2019 |
| OGO Solutions Limited | Real estate | 2024 |
| OH Good Evans Ltd | Administrative & support | 2024 |
| OIL Trading & Shipping Limited | Administrative & support | 1996 |
| Okkia Group Ltd | Manufacturing | 2026 |
| Old Barn 1 Limited | Professional & scientific | 2024 |
| Old Childsbridge Lane Woodland Limited | Agriculture & fishing | 2011 |
| Old School Court (Sevenoaks) Flat Maintenance Limited | Households | 1982 |
| Old Stone & Timber Ltd | Construction | 2025 |
| Oldstead Properties Limited | Financial services | 2022 |
| Olevi International Ltd | Education | 2013 |
| Olevi Limited | Extraterritorial | 2009 |
| Olivebox Group Limited | Hospitality | 2015 |
| Olivia Engelen Ltd | Professional & scientific | 2014 |
| Olivia Mesher Executive Support Ltd | Administrative & support | 2026 |
| Olivias Shop Ltd | Professional & scientific | 2023 |
| Ollie's Breakfast Bar Ltd | Hospitality | 2024 |
| Ollitec Limited | Wholesale & retail | 2019 |
| Ology (South East) Limited | Professional & scientific | 2006 |
| OLY Autos Ltd | Wholesale & retail | 2022 |
| Olyf Ltd | Wholesale & retail | 2025 |
| Olympus Bathrooms Essex Limited | Wholesale & retail | 2023 |
| Olympus Bathrooms Limited | Wholesale & retail | 2018 |
| Olympus Estate Agents Limited | Real estate | 2021 |
| Omaz Ltd | Construction | 2018 |
| OMF Group Ltd | Other services | 2023 |
| Omori Ltd | Professional & scientific | 2021 |
| ON Brand London Limited | Professional & scientific | 2023 |
| ON Y VA Films Limited | Information & communication | 2018 |
| ON Your SIX Ltd | Information & communication | 2017 |
| Onaya Science Limited | Professional & scientific | 2023 |
| One Export Import Ltd | Wholesale & retail | 2025 |
| One Media and Creative UK Limited | Information & communication | 2005 |
| One Mind Media Ltd | Arts & recreation | 2021 |
| One Tree Hill Holdings Ltd | Professional & scientific | 2023 |
| One Twenty Management Limited | Households | 2003 |
| One Warrior Ltd | Financial services | 2020 |
| One Warwick Park Limited | Hospitality | 2007 |
| Onesource Group Ltd | Administrative & support | 2026 |
| ONI Misi Limited | Professional & scientific | 2023 |
| Online Stores Europe Ltd | Wholesale & retail | 2009 |
| Onlinechoices Ltd | Wholesale & retail | 2026 |
| Ontrack Therapeutics Ltd | Professional & scientific | 2021 |
| Opal Coaching Ltd | Education | 2024 |
| Opal Group Limited | Wholesale & retail | 2001 |
| Opal Refurbishment Projects Limited | Real estate | 2010 |
| Opcon Services Limited | Professional & scientific | 2018 |
| Open Architecture Management Ltd | Professional & scientific | 2012 |
| Open Engineering Ltd | Professional & scientific | 2018 |
| Opendata Limited | Administrative & support | 2007 |
| Openslot Property Management Limited | Real estate | 1988 |
| Opsman Consulting Limited | Information & communication | 2013 |
| Optimal Organisational Outcomes LLP | | 2021 |
| Optimus Contractors Ltd | Construction | 2022 |
| Opulent Mindset Ltd | Administrative & support | 2025 |
| Orange Butterfly Limited | Professional & scientific | 2022 |
| Orange Lives Limited | Wholesale & retail | 2023 |
| Orangehexic Limited | Professional & scientific | 2022 |
| Orbis Developments (Sussex) Ltd | Construction | 2021 |