| O-Negativ Limited | Construction | 2017 |
| O.P.S Heating & Cooling Ltd | Construction | 2026 |
| OA Produced IT Ltd | Arts & recreation | 2024 |
| Oak Avenue Ltd | Professional & scientific | 2015 |
| Oak Cash & Carry Limited | Wholesale & retail | 2000 |
| Oakenwood Properties Ltd | Real estate | 2023 |
| Oakenwood Properties Partnership LLP | | 2023 |
| Oakland Property Ltd | Real estate | 2023 |
| Oakleigh Green Midlands Limited | Agriculture & fishing | 2017 |
| Oakridge Getaways Ltd | Real estate | 2025 |
| Oakridge Heating Co Ltd | Construction | 2025 |
| Oakwood Management Consultancy Limited | Real estate | 2016 |
| OBA Couriers Ltd | Transport & logistics | 2025 |
| Observia Limited | Information & communication | 2016 |
| OCD Emmett's Decorating Ltd | Construction | 2022 |
| Octave Agency Ltd | Information & communication | 2018 |
| Oculus Wealth Management (Oxfordshire) Limited | Financial services | 2018 |
| Odyssey Theatrical Productions Ltd | Arts & recreation | 2010 |
| Oemeta (UK) Limited | Manufacturing | 1997 |
| OFF the Page Productions Ltd | Information & communication | 2004 |
| Office Temps UK Ltd | Administrative & support | 2024 |
| OGF Electrical Ltd | Construction | 2024 |
| OGS Banbury Limited | Hospitality | 2024 |
| OJO Consulting Limited | Other services | 2024 |
| OJO Property Ltd | Real estate | 2026 |
| Olde England Roofs Ltd | Construction | 2026 |
| Older Persons Living Ltd | Construction | 2021 |
| Olive & Greene Limited | Manufacturing | 2020 |
| Olphine Holdings Limited | Other services | 2019 |
| Omagate Healthcare Consultancy Ltd | Health & social care | 2021 |
| Omagate Healthcare Professionals Ltd | Health & social care | 2025 |
| Omega Flex Limited | Manufacturing | 2000 |
| Omlet Limited | Manufacturing | 2004 |
| Omnia Gyms Limited | Arts & recreation | 2025 |
| ON HO 828 Limited | Hospitality | 2024 |
| ON Line Broadcasting Limited | Administrative & support | 1998 |
| Oncue Event Services Ltd | Administrative & support | 2024 |
| One MAN Band Ltd. | Wholesale & retail | 2001 |
| One of 22 Limited | Manufacturing | 2024 |
| Onea Motors Limited | Wholesale & retail | 2026 |
| Onefiftynine Ltd | Administrative & support | 2005 |
| Oneg Invest Ltd | Real estate | 2024 |
| Oneresource Virtual Assistants Limited | Administrative & support | 2015 |
| Oneupki Ltd | Wholesale & retail | 2018 |
| Onho 168 Limited | Hospitality | 2026 |
| Onhooxon Limited | Hospitality | 2025 |
| Online Motorsport Solutions Ltd | Other services | 2011 |
| Onside Coaching Cotswolds Ltd | Education | 2018 |
| Ontogeny Ltd | Professional & scientific | 2022 |
| Onyx Therapy Ltd | Health & social care | 2023 |
| Opal Fine Art Ltd | Wholesale & retail | 2008 |
| Opalescent Ltd | Information & communication | 2020 |
| Opliciti Ltd | Information & communication | 2026 |
| Optiedge Ltd | Education | 2024 |
| Option Development Limited | Information & communication | 2013 |
| Optivis Limited | Arts & recreation | 2026 |
| Optronica Limited | Manufacturing | 2014 |
| Opus Property Services Limited | Construction | 2009 |
| Opus Property Solutions Ltd | Construction | 2025 |
| Orbit Mechanical Ltd | Professional & scientific | 2025 |
| Orca Management Services Ltd | Professional & scientific | 2018 |
| Orchard Cloud Projects Ltd | Wholesale & retail | 2025 |
| Organic Packaging Ltd | Extraterritorial | 2000 |
| Origin Wine Limited | Wholesale & retail | 2004 |
| Originalis Ltd | Extraterritorial | 2025 |
| Orlando Studios Ltd. | Arts & recreation | 2000 |
| ORN (INT) Limited | Wholesale & retail | 2010 |
| Osana Procurement Ltd | Administrative & support | 2024 |
| Osborne Mechanical Services Limited | Construction | 2019 |
| Osborne Plumbing & Heating Services Ltd | Construction | 2017 |
| Oscorp Enterprises Ltd | Professional & scientific | 2023 |
| Osprey Estates Limited | Real estate | 2011 |
| Osprey Sports Limited | Wholesale & retail | 2018 |
| Ostrich and Rock Art Ltd | Wholesale & retail | 2025 |
| OTD Logistics Ltd | Transport & logistics | 2026 |
| Otfauto Ltd | Wholesale & retail | 2026 |
| OTI Consulting Services Limited | Professional & scientific | 2022 |
| OTI Holdings Ltd | Financial services | 2022 |
| Otmoor Partners Limited | Information & communication | 2012 |
| Otter Hockey Limited | Wholesale & retail | 2020 |
| Ottilie Property Ltd | Real estate | 2022 |
| OUR BUS Bartons Ltd | Transport & logistics | 2016 |
| OUROBOROS-360 Consultancy Services Limited | Information & communication | 2018 |
| Outdoor MAX Limited | Wholesale & retail | 2005 |
| Ovation Commercial Services Ltd | Wholesale & retail | 2016 |
| Overfield Industrial Estate Management Company Limited | Real estate | 2018 |
| Overthorpe Garage Limited | Wholesale & retail | 2009 |
| Overthorpe Properties Limited | Real estate | 2022 |
| Overthorpe Road Management Company Limited | Real estate | 2004 |
| Ovilum Ltd | Transport & logistics | 2019 |
| Owlets Consulting Ltd | Professional & scientific | 2011 |
| Owls Roost Ltd | Hospitality | 2023 |
| Ownership Ltd | Wholesale & retail | 2021 |
| OX Architecture & Interiors Ltd | Professional & scientific | 2018 |
| OX15 Services Limited | Hospitality | 2018 |
| OX16 UK Ltd | Wholesale & retail | 2023 |
| Oxaedes Property Ltd | Real estate | 2026 |
| Oxbridge Construction Ltd | Construction | 2020 |
| Oxbridge Global Academy Ltd | Administrative & support | 2022 |
| Oxford (Online) Bidco Limited | Financial services | 2024 |