| Oakhill Engineering and Surveying Limited | Construction | 2013 |
| Oakhurst Living Limited | Wholesale & retail | 2022 |
| Oakland Construction Limited | Construction | 1991 |
| Oaklands Business Park Management Limited | Real estate | 1999 |
| Oaklands Carpentry and Build Ltd | Construction | 2023 |
| Oaklands Gilwern Limited | Transport & logistics | 2004 |
| Oaklands Professional Services Ltd | Professional & scientific | 2014 |
| Oaklane Limited | Professional & scientific | 2003 |
| Oakleaf Wellness Centre Ltd | Extraterritorial | 2020 |
| Oakleaze Barbers Limited | Other services | 2023 |
| Oakleigh Associated Properties Limited | Real estate | 2007 |
| Oakleigh Group Ltd | Professional & scientific | 2023 |
| Oakleigh Leigh Woods Bristol (Management) Limited | Households | 1986 |
| Oakleigh Management Consulting Limited | Information & communication | 2017 |
| Oakley Creations Limited | Construction | 2013 |
| Oakley Green Conservatories Limited | Construction | 2004 |
| Oakley Green Design & Build Ltd | Construction | 2022 |
| Oakley Green Farms Limited | Agriculture & fishing | 2021 |
| Oakley Green Ltd | Manufacturing | 2023 |
| Oakley Healthcare Services Ltd | Health & social care | 2025 |
| Oakley Logistics Limited | Transport & logistics | 2026 |
| Oakleyandrews Developments Ltd | Construction | 2023 |
| Oakmire Limited | Real estate | 2018 |
| Oakridge Consultants Limited | Real estate | 2023 |
| Oakridge Financial Planning Limited | Financial services | 2002 |
| Oakridge Services Limited | Real estate | 2003 |
| Oaks Catering Ltd | Wholesale & retail | 2019 |
| Oaks Limited | Information & communication | 2004 |
| Oakside Limited | Real estate | 2015 |
| Oakstone Carpentry & Construction Limited | Construction | 2026 |
| Oakstone Management Ltd | Administrative & support | 2026 |
| Oaktree (Clevedon) Limited | Health & social care | 2004 |
| Oaktree Holdings South West Ltd | Construction | 2021 |
| Oakwood Airport Parking Limited | Transport & logistics | 2014 |
| Oakwood Building & Roofing Ltd | Construction | 2018 |
| Oakwood Media Holdco Limited | Administrative & support | 2012 |
| Oakwood Meet and Greet Ltd | Transport & logistics | 2026 |
| Oakwood Project Consulting Ltd | Professional & scientific | 2016 |
| Oakwood Staff Solutions Limited | Administrative & support | 2004 |
| Oakwood Training Solutions Limited | Administrative & support | 2011 |
| Oandl Dili Ventures Limited | Health & social care | 2025 |
| Oase Outdoors Limited | Wholesale & retail | 2011 |
| Oasis Accountancy Limited | Professional & scientific | 2025 |
| Oasis Bathrooms & Heating Limited | Construction | 2011 |
| Oasis Dental Care (Central) Limited | Health & social care | 1952 |
| Oasis Dental Care (Southern) Holdings Limited | Extraterritorial | 1993 |
| Oasis Dental Care (Southern) Limited | Health & social care | 1954 |
| Oasis Dental Care Limited | Health & social care | 1950 |
| Oasis Health Care Services Limited | Health & social care | 2019 |
| Oasis Healthcare Limited | Professional & scientific | 1996 |
| Oasis Kingdom Ltd | Information & communication | 2025 |
| Oasis Pool Renovations Limited | Construction | 2026 |
| Oasys Services Limited | Professional & scientific | 1988 |
| OAT Float Ltd | Wholesale & retail | 2021 |
| Oates Catering Ltd | Hospitality | 2025 |
| Oatley Recruitment & Consultancy Limited | Administrative & support | 2015 |
| Oaza Kobiecosci C.I.C. | Other services | 2025 |
| OB Farms & Produce Limited | Administrative & support | 2026 |
| OBC Testing Solutions Ltd | Information & communication | 2022 |
| OBE1 Ltd | Professional & scientific | 2016 |
| Obelia Limited | Wholesale & retail | 2019 |
| Obelisk Music Limited | Information & communication | 2014 |
| Oberi.Co Ltd | Information & communication | 2014 |
| Obertal Ltd | Professional & scientific | 2020 |
| Obesity and Metabolism Clinic Ltd | Health & social care | 2025 |
| Obhanrose Consulting Ltd | Administrative & support | 2018 |
| Obiadazie Care Limited | Health & social care | 2022 |
| Obis Capital Limited | Real estate | 2026 |
| Object Soup Ltd | Professional & scientific | 2024 |
| Objection Experts Limited | Professional & scientific | 2023 |
| Objective 1 Limited | Information & communication | 2011 |
| Objective Release Limited | Information & communication | 2017 |
| Oblix Advisors LLC Limited | Financial services | 2022 |
| OBP Accounting and Finance Limited | Professional & scientific | 2023 |
| OBS Management Services Limited | Households | 2021 |
| Obsidian Accountancy Limited | Professional & scientific | 2015 |
| Obsidian Coffee Ltd | Hospitality | 2023 |
| Obsidian Construction (SW) Ltd | Construction | 2022 |
| Obsidian Homes Ltd | Real estate | 2016 |
| Obsurvant Ltd | Professional & scientific | 2021 |
| Obykiama's Ltd | Health & social care | 2022 |
| OC Analytics Ltd | Information & communication | 2020 |
| OC Interiors Ltd | Financial services | 2019 |
| OCA Health Limited | Health & social care | 2024 |
| OCB 888 Limited | Hospitality | 2019 |
| Occasion Cakes and Catering Ltd | Wholesale & retail | 2018 |
| Occg Limited | Arts & recreation | 2025 |
| Occupy (CRE) Limited | Real estate | 2022 |
| Ocean & Mountain Explorers Limited | Financial services | 2018 |
| Ocean 29 Ltd | Wholesale & retail | 2021 |
| Ocean Aircraft Ltd | Manufacturing | 2023 |
| Ocean and Coastal Futures Ltd | Professional & scientific | 2022 |
| Ocean Blue Software Limited | Information & communication | 1991 |
| Ocean Breeze HR Limited | Administrative & support | 2022 |
| Ocean Catch SW Limited | Hospitality | 2018 |
| Ocean City Chen Ltd | Hospitality | 2018 |
| Ocean Dreams Ltd | Hospitality | 1998 |
| Ocean Estate Agents Limited | Real estate | 1995 |
| Ocean Financial Services Limited | Financial services | 1995 |
| Ocean Horizon Ltd | Real estate | 2025 |