| Oakfield Park Gardens Management Limited | Households | 1996 |
| Oakford Court (Henley on Thames) Management Company Limited | Households | 2017 |
| Oaklands Court Residents Limited | Households | 1996 |
| Oakley Court Acquisitions Unlimited | | 2006 |
| Oakley Grange Management Company Limited | Households | 1994 |
| Oakley Green (Lavant) Management Company Limited | Households | 2023 |
| Oakley Green Cars Limited | Wholesale & retail | 2025 |
| Oakley Mews Management Limited | Households | 1995 |
| Oakley Mews Residents Management Company Limited | Households | 2017 |
| Oakley Waste Limited | Wholesale & retail | 2024 |
| Oakmont House Creative Limited | Professional & scientific | 2020 |
| Oblbr Ltd | Professional & scientific | 2012 |
| Ocean Liner Trading Limited | Other services | 1991 |
| OCR Recruitment Solutions Ltd | Administrative & support | 2016 |
| Oduco Limited | Construction | 2012 |
| OFF the Cuff Productions Ltd | Information & communication | 2010 |
| Offenders Group Ltd | Wholesale & retail | 2024 |
| Officepal Ltd | Mining & quarrying | 2011 |
| Offpaper Studios Ltd | Information & communication | 2025 |
| Ogc-Uk Limited | Professional & scientific | 2020 |
| Ogilviedavies Ltd | Professional & scientific | 2008 |
| OH My YOG! Limited | Wholesale & retail | 2025 |
| OH Parsons LLP | | 2013 |
| Ojsr Limited | Professional & scientific | 2010 |
| Okaeri Homes Ltd | Real estate | 2019 |
| Old Chapel Property Consultants Limited | Administrative & support | 2014 |
| Old Windsor Logistics Ltd | Transport & logistics | 2018 |
| Oldham Trading Limited | Wholesale & retail | 2008 |
| Oldlands Grange (Ripley) Management Company Ltd | Households | 2022 |
| Olive & Fred's Flip Limited | Real estate | 2023 |
| Olive & Fred's R2R Limited | Real estate | 2023 |
| Oliver Smith Transport Limited | Transport & logistics | 2006 |
| Ollerton Banks Property Ltd | Construction | 2005 |
| Olly Williams Property Ltd | Real estate | 2023 |
| Omniroute Solutions Group Ltd | Transport & logistics | 2025 |
| Omnium CONSULTANTS2GO Limited | Information & communication | 2011 |
| Omologato Limited | Wholesale & retail | 2015 |
| ON Site Spray Ltd | Construction | 2016 |
| On-Demand Marketing Services Ltd | Information & communication | 2018 |
| One 2 One Autos Ltd | Wholesale & retail | 2019 |
| One Four Design Signs & Graphics Ltd | Arts & recreation | 2014 |
| One in Millions Ltd. | Construction | 2019 |
| One MO Mile Limited | Arts & recreation | 2024 |
| One Nine Assets Limited | Real estate | 2024 |
| One Nine Corporate Limited | Real estate | 2025 |
| One Nine Finances Limited | Financial services | 2021 |
| One Property Recruitment Ltd | Administrative & support | 2020 |
| One Stop Render Supplies Ltd | Wholesale & retail | 2025 |
| Onehemwood Limited | Financial services | 2021 |
| Onetalent Ltd | Administrative & support | 2020 |
| Online Construction Consultants Ltd | Construction | 2025 |
| Only Earth Ltd | Professional & scientific | 2025 |
| Onsight Cloud Solutions Ltd | Financial services | 2022 |
| Onslow Design and Build Limited | Real estate | 2024 |
| OP & DL Limited | Real estate | 2018 |
| Opal Wealth Limited | Professional & scientific | 2026 |
| Open Shutter Productions Limited | Information & communication | 1989 |
| Open Therapies Ltd | Health & social care | 2019 |
| Opium Aesthetics Ltd | Other services | 2025 |
| Oppidan Residents Company Limited | Households | 2005 |
| Optima Interiors Ltd | Construction | 2013 |
| Optima Wellness Limited | Wholesale & retail | 2022 |
| Optimus Intelligence Limited | Information & communication | 2025 |
| Opus Developments International Ltd | Real estate | 2023 |
| Oran AJ Limited | Hospitality | 2025 |
| Orange Life Solutions Ltd | Professional & scientific | 2016 |
| Oranges & Lemons Services Ltd | Administrative & support | 2026 |
| Orara Limited | Information & communication | 2016 |
| Orchard Avenue Limited | Households | 1998 |
| Orchard LEA (Winkfield) Management Company Limited | Households | 2019 |
| Organic Festive Nature Limited | Wholesale & retail | 2023 |
| Oriel Consult Limited | Professional & scientific | 2013 |
| Orion Aircraft UK and Europe Limited | Manufacturing | 2026 |
| Orion PSP Ltd | Wholesale & retail | 2023 |
| Osborne Court Residents Association Limited | Households | 1959 |
| Osborne Road Management Company (Windsor) Limited | Households | 1981 |
| Osborne Road Management Company Limited | Households | 1997 |
| Osteolabs UK Limited | Wholesale & retail | 2021 |
| Otex Electrical Services Ltd | Construction | 2015 |
| Other Space Arts CIC | Hospitality | 2024 |
| Otsuka Pharmaceutical Europe Ltd. | Professional & scientific | 1997 |
| Otsuka Pharmaceuticals (U.K.) Ltd. | Wholesale & retail | 1998 |
| Ottershaw Properties Ltd | Real estate | 2016 |
| OUR Coisa Ltd | Real estate | 2025 |
| OUT of This World Productions Limited | Information & communication | 2026 |
| Outdoor Owls Ltd | Health & social care | 2019 |
| Outside the Box Services Limited | Arts & recreation | 2018 |
| Oven Star Ltd | Hospitality | 2024 |
| Overview Press Limited | Information & communication | 2003 |
| OWA (UK) Limited | Manufacturing | 1982 |
| Ownership Partners Limited | Professional & scientific | 2024 |
| Oxford & Cambridge Banbury Partnership | | 1989 |
| Oxford & Cambridge Brackley Partnership | | 1989 |
| Oxford & Cambridge Uxbridge Partnership | | 1989 |
| Oxford Spires Hotel Opco Limited | Hospitality | 2018 |
| Oxford Teachings Ltd | Education | 2022 |
| Oxford Thames Hotel Opco Limited | Hospitality | 2018 |
| OXFORD164 RTM Company Ltd | Hospitality | 2019 |
| OXO21 Ltd | Hospitality | 2023 |
| O’Neil’S Property Surveyors Limited | Professional & scientific | 2023 |