| Oceanbird Ltd | Other services | 2019 |
| Oceet Ltd | Professional & scientific | 2023 |
| Ochre BIO Ltd | Professional & scientific | 2019 |
| OCK Mill Management Company Limited | Households | 1988 |
| Ockham Court Properties Ltd | Households | 2023 |
| Ocla Ltd | Professional & scientific | 2015 |
| Oconnor Property Ltd | Construction | 2021 |
| Octillion Ltd | Health & social care | 2019 |
| Octopus Physiotherapy Ltd | Health & social care | 2014 |
| Oculus Sports Limited | Manufacturing | 2016 |
| ODC Europe (NTL) Limited | Professional & scientific | 2013 |
| Oddly Specific Ltd | Professional & scientific | 2025 |
| Odqa Renewable Energy Technologies Limited | Manufacturing | 2017 |
| Odubbs Creative Limited | Professional & scientific | 2023 |
| Odumalife Limited | Arts & recreation | 2018 |
| Odyssey Canine Academy Ltd | Other services | 2023 |
| OEH Designs Ltd | Administrative & support | 2022 |
| Oelt Limited | Information & communication | 1984 |
| Ofac Limited | Public administration | 2023 |
| OFF the Hook (Oxford) Limited | Hospitality | 2014 |
| Office on Cloud 9 Ltd | Information & communication | 2013 |
| Offices.Now Ltd | Real estate | 2000 |
| Officium Limited | Professional & scientific | 2000 |
| OG Private Limited | Wholesale & retail | 2026 |
| Ogbeni Global Ltd | Health & social care | 2019 |
| Ogglio UK Ltd | Manufacturing | 2020 |
| Ogilvie Oxford Ltd | Professional & scientific | 2002 |
| OGM Group Holdings Limited | Manufacturing | 2025 |
| OGM Moulding Limited | Manufacturing | 2025 |
| OH My Stars! Limited | Hospitality | 2015 |
| OH Ventures Ltd | Professional & scientific | 2019 |
| Ohana Capital Ltd | Financial services | 2017 |
| Ohana Community Holding CIC | Real estate | 2019 |
| OHC Developments Limited | Administrative & support | 2019 |
| OHR Consulting Limited | Professional & scientific | 2025 |
| OI Interiors Oxford Ltd | Manufacturing | 2013 |
| OIC Oxford Ltd | Professional & scientific | 2023 |
| Oicd Emic Limited | Education | 2020 |
| OIE Sportswear Limited | Manufacturing | 2026 |
| Oies Services Limited | Professional & scientific | 2019 |
| Oisa Ltd | Professional & scientific | 2022 |
| Oise Holdings Limited | Professional & scientific | 2006 |
| Oise Limited | Education | 1977 |
| Ojas Immune Limited | Professional & scientific | 2024 |
| Ojrl Management Limited | Professional & scientific | 2013 |
| OK Energy Limited | Mining & quarrying | 2010 |
| OKA Properties Management Ltd | Real estate | 2020 |
| Okkura Limited | Administrative & support | 2024 |
| Okrag Ltd | Information & communication | 2024 |
| Oksanalovas Ltd | Administrative & support | 2025 |
| Okyoobell Limited | Administrative & support | 2017 |
| OL4ALL Limited | Education | 2023 |
| Oladol Engineering and Management Consultants UK Limited | Construction | 2009 |
| Olara Ltd | Construction | 2022 |
| Olawhykay Ltd | Health & social care | 2022 |
| Olawuyi Racett Nigeria Ltd. | Professional & scientific | 2023 |
| Old Farmhouse (UK) LLP | | 2013 |
| Old Ford Road (Tower) Management Company Limited | Real estate | 2011 |
| Old Orchard / Robynswood Management Company Ltd | Households | 2020 |
| Old TOM Oxford Ltd | Hospitality | 2016 |
| Olive Tree Property Group Ltd | Real estate | 2025 |
| Olivemoud Limited | Health & social care | 2014 |
| Oliver Greig Consulting Ltd | Professional & scientific | 2025 |
| Oliver Legge Furniture Limited | Manufacturing | 2014 |
| Oliver Tedder Consulting Ltd | Professional & scientific | 2024 |
| Olivia's Greek Food Limited | Hospitality | 2021 |
| Olley Limited | Information & communication | 1997 |
| Olly Huggins Golf Ltd | Arts & recreation | 2022 |
| Olma Services Ltd | Construction | 2024 |
| Olof Limited | Professional & scientific | 2023 |
| Olsa Limited | Professional & scientific | 2024 |
| Olti Properties Ltd | Real estate | 2021 |
| Olymnos Ltd | Wholesale & retail | 2021 |
| Omar Letting Limited | Real estate | 2025 |
| Omass Therapeutics Limited | Professional & scientific | 2016 |
| Ombs Investments Limited | Real estate | 2009 |
| Ombs Limited | Education | 2007 |
| Ombs Oxford Limited | Education | 2025 |
| OMC Property Ltd | Extraterritorial | 2015 |
| Omcrg Limited | Real estate | 2003 |
| Omega Acquisition Bidco Limited | Professional & scientific | 2015 |
| Omega Education Ltd | Education | 2026 |
| Omega Newco SUB I Limited | Professional & scientific | 2015 |
| Omega Newco SUB II Limited | Professional & scientific | 2015 |
| Omega Pro Cleaning Ltd | Administrative & support | 2025 |
| Omic Consulting Ltd | Professional & scientific | 2014 |
| Omics Consultancy Ltd | Professional & scientific | 2019 |
| Omni Interactive Group Ltd | Information & communication | 2026 |
| Omni-Natal Limited | Health & social care | 2017 |
| Omnima Limited | Manufacturing | 2007 |
| Omnitrax Ltd | Wholesale & retail | 2025 |
| ON 1989 Estates Limited | Real estate | 2023 |
| ON a Tangent Ltd | Information & communication | 2025 |
| ON Assessments Ltd | Health & social care | 2023 |
| ON Call CMO Limited | Professional & scientific | 2022 |
| ON Higher Grounds Ltd | Professional & scientific | 2017 |
| ON OFF Europe Ltd | Professional & scientific | 2015 |
| One 80 Flat Management Limited | Real estate | 2009 |
| One Limited | Professional & scientific | 2000 |
| One System MGT Ltd | Professional & scientific | 2024 |