| O.J. Gyms Ltd | Arts & recreation | 2018 |
| OAE (SW) Ltd | Construction | 2022 |
| OAE Autos Ltd | Wholesale & retail | 2022 |
| Oak Business Units Management Company Limited | Households | 2019 |
| Oak First Investments Limited | Wholesale & retail | 2015 |
| Oak Folk Devon Ltd | Construction | 2023 |
| Oak House (Exeter) Ltd | Health & social care | 2008 |
| Oak Tree Business Park (Kingskerswell) Management Company Limited | Households | 2022 |
| Oak Tree Farm Estate Limited | Hospitality | 2021 |
| Oakalls Consultancy Limited | Professional & scientific | 2008 |
| Oakbridge Associates Ltd | Information & communication | 2026 |
| Oakdene (Lifton) Management Company Limited | Households | 2024 |
| Oakdene Barns Limited | Households | 1990 |
| Oakfield Project Services Limited | Construction | 2018 |
| Oakhayes Limited | Construction | 2022 |
| Oakminster (SW) Limited | Administrative & support | 2009 |
| Oaks & Pitts Farm Limited | Agriculture & fishing | 2011 |
| Oakshaw Property Ltd | Real estate | 2015 |
| Oakshield Construction Ltd | Other services | 2017 |
| Oakview Land Limited | Construction | 2005 |
| Oakwood Accommodation Ltd | Real estate | 2021 |
| Oakwood Apartments Ltd | Real estate | 2016 |
| Oakwood Media Group Limited | Professional & scientific | 2006 |
| Oasis Flafel and Grill Limited | Wholesale & retail | 2023 |
| Oasis Invest Uzbekistan Limited | Financial services | 2021 |
| Oasis Property South West Limited | Real estate | 2014 |
| OAT & Angel Healthcare Limited | Health & social care | 2025 |
| OB Consultancy Limited | Administrative & support | 2016 |
| OB Innovations Limited | Information & communication | 2019 |
| Obenshaw Limited | Administrative & support | 1979 |
| Obsidian Black Ltd | Wholesale & retail | 2017 |
| Obsolete (Devon) Ltd | Wholesale & retail | 2024 |
| Obvis Limited | Information & communication | 2000 |
| Occams Capital Ltd | Information & communication | 2023 |
| Occamsec Ltd | Information & communication | 2015 |
| Occupational Health Assessment Service Ltd | Health & social care | 2022 |
| Occupational Therapy Devon Ltd | Other services | 2013 |
| Ocean Activist Ltd | Wholesale & retail | 2025 |
| Ocean Construction Cornwall Limited | Agriculture & fishing | 2019 |
| Ocean Physio and Rehab Ltd | Health & social care | 2008 |
| Ocean Wild Fish Bar Ltd | Hospitality | 2025 |
| Oceanbarefoot Limited | Professional & scientific | 2006 |
| Oceanic Wave Ltd | Professional & scientific | 2024 |
| Ochresoft Technologies Limited | Information & communication | 1998 |
| Octavia Group Ltd | Financial services | 2025 |
| OD Partners Ltd | Professional & scientific | 2009 |
| ODD KIN Coffee Roasters Limited | Hospitality | 2015 |
| Oddfellows Ltd | Hospitality | 2018 |
| Oddicombe Beach Resort Limited | Hospitality | 2014 |
| Odhams Contracts Limited | Construction | 2013 |
| Odin Estates Limited | Construction | 2008 |
| Odin's Card Shop Ltd | Wholesale & retail | 2023 |
| ODS Couriers Ltd | Transport & logistics | 2018 |
| OEA Laboratories Limited | Manufacturing | 2006 |
| OFF Grid Social Limited | Professional & scientific | 2025 |
| OFF Grid Systems (Europe) Ltd | Construction | 2015 |
| Officelabs Limited | Information & communication | 2012 |
| Official Ltd | Wholesale & retail | 2025 |
| Offshaw Wind Ltd | Construction | 2024 |
| OGB Toekoms Ltd | Health & social care | 2026 |
| Ogwell Brook Management Company Limited | Households | 2010 |
| OH My Kimchi Ltd | Hospitality | 2024 |
| Oikos LAB Ltd | Wholesale & retail | 2024 |
| Oilmill Property Services Ltd | Real estate | 2021 |
| OJT Properties Ltd | Real estate | 2018 |
| OK Vape Limited | Extraterritorial | 2018 |
| Okehampton East Management Company Limited | Households | 2017 |
| OL Realisations (2024) Limited | Construction | 2007 |
| Old Brewhouse Management Company (Bath) Limited | Real estate | 1991 |
| Old Ferry Inn (Bodinnick) Ltd | Hospitality | 2021 |
| Old Number 3 LLP | | 2009 |
| Old Oak Property and Letting Ltd | Real estate | 2024 |
| Old Postern Holdings Limited | Real estate | 2026 |
| Old School Wookey Management Company Ltd | Administrative & support | 2010 |
| Old Smithy (Rockbeare) Management Limited | Households | 2016 |
| Old Timers Limited | Hospitality | 2010 |
| Old Trafford Construction Limited | Construction | 2007 |
| Oldcliff Limited | Health & social care | 2015 |
| Oldhammer Miniature Company Limited | Manufacturing | 2016 |
| Oldmare Management Company Limited | Households | 1994 |
| Oldrieve Construction Ltd | Construction | 2019 |
| Olds Demolition & Salvage Limited | Construction | 2003 |
| Olek Polish Shop Ltd | Wholesale & retail | 2017 |
| Olford Properties Limited | Real estate | 2024 |
| Olimba Ltd | Real estate | 2025 |
| Olive and Lily Co Ltd | Wholesale & retail | 2026 |
| Olive NET Limited | Information & communication | 2003 |
| Olivelle & Associates Limited | Health & social care | 2007 |
| Oliver House (Poundbury) Management Company Limited | Real estate | 2025 |
| Olivia Frear Ltd | Professional & scientific | 2017 |
| Olivier Meylan Ltd | Manufacturing | 2020 |
| Ollie Amos Ltd | Professional & scientific | 2025 |
| Ollika Limited | Health & social care | 2025 |
| Olly & Tom's Ltd | Manufacturing | 2021 |
| OLO Real Estate Limited | Financial services | 2015 |
| Olsoph Ltd | Real estate | 2018 |
| Olve Services Limited | Professional & scientific | 2026 |
| Olympia Property Ltd | Real estate | 2023 |
| Olympus Bess I Ltd | Energy | 2024 |
| Olympus EV Limited | Energy | 2024 |