| O&K Heating & Plumbing Ltd | Construction | 2023 |
| O'Reilly Properties Limited | Real estate | 2016 |
| O'Scurlog International Limited | Transport & logistics | 2015 |
| Oak Leaf Designs Ltd | Information & communication | 2010 |
| Oak Tree Obstetrics Limited | Health & social care | 2024 |
| Oaka Living Ltd | Real estate | 2025 |
| Oasis ST Helens Ltd | Health & social care | 2026 |
| Oast House Consulting Education and Finance Ltd | Professional & scientific | 2026 |
| Oast House Management Company Limited(The) | Households | 1980 |
| Oast Studios Kent CIC | Arts & recreation | 2025 |
| Oaten Ltd | Wholesale & retail | 2026 |
| Oatley & O'Connor Ltd | Real estate | 2021 |
| OB Micro Limited | Wholesale & retail | 2006 |
| OBA Health Metrics Limited | Professional & scientific | 2025 |
| Obee Automotive Limited | Wholesale & retail | 2024 |
| Obee Estates Limited | Real estate | 2004 |
| Obee Holdings Limited | Financial services | 2022 |
| Obee Properties Limited | Real estate | 2012 |
| Obscuron Ltd | Hospitality | 2025 |
| Occasio Developments (Littlestone) Ltd | Construction | 2025 |
| Occasio Developments Ltd | Construction | 2023 |
| Occupy Properties Ltd | Construction | 2025 |
| OCD Car Valeting & Detailing Ltd | Wholesale & retail | 2021 |
| Ocean Soul Wellbeing Limited | Health & social care | 2026 |
| Oceanic Cuts Ltd | Wholesale & retail | 2023 |
| Ocito Limited | Arts & recreation | 2003 |
| Ocklynge Properties Limited | Households | 1967 |
| Ocok (Overseas Chinese of Kent) Ltd | Wholesale & retail | 2022 |
| October Properties Limited | Real estate | 2024 |
| Octoplus Swim School Ltd | Education | 2025 |
| Octopus Global Ltd | Information & communication | 2022 |
| Odiam Poultry Limited | Agriculture & fishing | 1976 |
| Odide Ltd | Wholesale & retail | 2023 |
| Odyssey & Bros Ltd | Other services | 2025 |
| OEC Management 6 Ltd | Construction | 2024 |
| Oefureys Ltd | Wholesale & retail | 2025 |
| Officium Familia Ltd | Financial services | 2025 |
| Offset Intimates Ltd | Wholesale & retail | 2021 |
| OG Canterbury Ltd | Hospitality | 2023 |
| OGI Property Rental Ltd | Real estate | 2023 |
| OH AYE Technology Limited | Information & communication | 2020 |
| OH Pharma Limited | Wholesale & retail | 2024 |
| Ohtani Technologies Limited | Information & communication | 2025 |
| Oida! Theatre Ltd | Arts & recreation | 2023 |
| OJS Consulting Ltd | Information & communication | 2018 |
| OKO Oluto (UK) Limited | Agriculture & fishing | 2020 |
| Oktoberfest Canterbury Ltd | Arts & recreation | 2023 |
| Old Forge Property Investments Limited | Real estate | 2016 |
| Old Park Court Management Company Limited | Households | 1986 |
| Old Renville Farm Management Limited | Professional & scientific | 2002 |
| Old School Services Canterbury Limited | Health & social care | 2012 |
| Old Town Hall Margate Limited | Real estate | 2022 |
| Old Weavers Restaurant Ltd | Hospitality | 2018 |
| Olive Grove Canterbury Limited | Hospitality | 2020 |
| Oliver Jones Limited | Administrative & support | 2012 |
| Olivia ROE Pathways Ltd | Education | 2025 |
| Olivia Ruella Limited | Administrative & support | 2020 |
| Oludamilola Olusile Ltd | Health & social care | 2023 |
| Oluto UK Holdings Limited | Financial services | 2022 |
| OM Studio & Cafe Ltd | Hospitality | 2025 |
| OMA OMA Ltd | Hospitality | 2024 |
| Omandea Community Care Ltd | Health & social care | 2026 |
| Omics Ltd | Professional & scientific | 2013 |
| Omnis RES Ltd | Manufacturing | 2023 |
| Omny Group Limited | Extraterritorial | 2024 |
| Omny Limited | Extraterritorial | 2024 |
| ON Architecture Limited | Professional & scientific | 2011 |
| ON Developments Limited | Extraterritorial | 2020 |
| ON JOB Clean Limited | Administrative & support | 2024 |
| ON My Own Business Ltd | Transport & logistics | 2021 |
| ON Planning Limited | Professional & scientific | 2025 |
| ON Star Taxi Limited | Transport & logistics | 2020 |
| ON Visualisation Limited | Professional & scientific | 2025 |
| One Call Electrical Services Ltd | Administrative & support | 2007 |
| One Canterbury Ltd | Construction | 2023 |
| One Pick Technologies Limited | Information & communication | 2025 |
| One Pound Lane Canterbury Ltd | Hospitality | 2014 |
| One Pound Lane Ltd | Hospitality | 2014 |
| One Square Limited | Professional & scientific | 2024 |
| One Tree Hill Properties (UK) Limited | Real estate | 2020 |
| Onethreesixteen Limited | Real estate | 2021 |
| Onit Systems Ltd | Information & communication | 2024 |
| Onith Group Ltd | Wholesale & retail | 2026 |
| Onroute Transport Group Ltd | Transport & logistics | 2026 |
| Ontime Cars Limited | Transport & logistics | 2018 |
| Onward Bioscience Ltd | Professional & scientific | 2021 |
| Onyx Housing Ltd | Construction | 2024 |
| Ookidon LLP | | 2025 |
| Opal Foundry Ltd | Information & communication | 2026 |
| Opeg Extras Ltd | Professional & scientific | 2014 |
| Open Controls Ltd | Professional & scientific | 2015 |
| Open Invoice Finance Limited | Financial services | 2013 |
| Opsis Solutions Limited | Manufacturing | 2025 |
| Opsmagic Limited | Information & communication | 2026 |
| Optimus Facilities Management Ltd | Administrative & support | 2017 |
| Optopod Limited | Manufacturing | 2005 |
| Opulent Skin Limited | Health & social care | 2024 |
| Opuna Ltd | Health & social care | 2023 |
| Orange Pebble Ltd | Health & social care | 2016 |
| Oraz Builders Ltd | Construction | 2026 |