| One Inspect Limited | Professional & scientific | 2016 |
| One IVY Developments Limited | Professional & scientific | 2025 |
| One Pipewellgate Ltd | Real estate | 2024 |
| One Smart Move Ltd | Real estate | 2020 |
| One Spot Shop Ltd | Wholesale & retail | 2025 |
| One Thirty 3 Limited | Arts & recreation | 2011 |
| One Unite Partnership Community Interest Company | Energy | 2017 |
| Oneg Foods Ltd | Hospitality | 2025 |
| Onexp Group Limited | Financial services | 2021 |
| Onifaaji Limited | Real estate | 2026 |
| Onix Lift and Escalator Services Ltd | Manufacturing | 2026 |
| Onlineology Ltd | Information & communication | 2008 |
| Ontie Development Ltd | Other services | 2021 |
| Onwards and Upwards North East Ltd | Professional & scientific | 2021 |
| Onyx Jewellery Limited | Manufacturing | 2025 |
| OOH LA LA Occasions and Events Ltd | Hospitality | 2026 |
| Open Door Lettings Ltd | Real estate | 2019 |
| Openweightless C.I.C. | Information & communication | 2018 |
| Operational Design & Solutions Ltd | Professional & scientific | 2021 |
| Opion Limited | Construction | 2005 |
| Opsr Offices Limited | Wholesale & retail | 2025 |
| Opti Composites Ltd | Manufacturing | 2012 |
| Optical RX Limited | Wholesale & retail | 2023 |
| Optima Paragon Ltd | Information & communication | 2026 |
| Optimal Decorating Contractors Limited | Construction | 2019 |
| Optimal Solutions NE Limited | Construction | 2021 |
| Optimate Consulting Ltd | Professional & scientific | 2016 |
| Optimise Cloud Ltd | Information & communication | 2023 |
| Optimised Limited | Professional & scientific | 2013 |
| Optimum Cricket Limited | Wholesale & retail | 2020 |
| Optimus Drive Ltd | Education | 2021 |
| Option 1 Vehicle Styling Ltd | Wholesale & retail | 2025 |
| ORA Brewing Company Ltd | Wholesale & retail | 2016 |
| Orand Limited | Wholesale & retail | 2025 |
| Orangutan ARB Ltd | Agriculture & fishing | 2020 |
| Orawsko Ltd | Wholesale & retail | 2022 |
| Orbis Community CIC | Arts & recreation | 2021 |
| Orbitwear Ltd | Administrative & support | 2018 |
| Orchard GAP Management Company Limited | Households | 2017 |
| Orderelite Limited | Real estate | 1996 |
| Organic Waste Solutions Limited | Professional & scientific | 2025 |
| Origin MS Ltd | Arts & recreation | 2024 |
| Origin Tech Ltd | Professional & scientific | 2019 |
| Original Fit OUT Limited | Construction | 2017 |
| Original Health and Wellness Ltd | Health & social care | 2022 |
| Original Investments Group Limited | Financial services | 2020 |
| Original Investments Limited | Real estate | 2020 |
| Original Joinery Limited | Construction | 2014 |
| Original Meal Across Ocean (Omao) Ltd | Hospitality | 2025 |
| Original PROP1 Limited | Real estate | 2024 |
| Original Student Living Limited | Households | 2024 |
| Orilog Ltd | Professional & scientific | 2024 |
| Orion Contract Services Ltd | Construction | 2019 |
| Orion Entertainments Limited | Professional & scientific | 1960 |
| ORM MOT Centre Ltd | Wholesale & retail | 2024 |
| Osbourne Foods Ltd | Hospitality | 2025 |
| OSO Hotwater Ltd | Other services | 1993 |
| Otaski Energy Solutions Ltd | Manufacturing | 2017 |
| Otlaris Cleaning & Facilities Management Limited | Administrative & support | 2024 |
| OTR Property Group Ltd | Real estate | 2018 |
| Otrcc S.T.E.P.S Management Services Ltd | Administrative & support | 2022 |
| Otter REC Ltd | Administrative & support | 2024 |
| Otter Talent Ltd | Administrative & support | 2024 |
| Otti Group Ltd | Information & communication | 2025 |
| OUR Children's World Group Ltd | Education | 2018 |
| OUR Northern Stars Limited | Health & social care | 2018 |
| OUR Small Garden Ltd | Wholesale & retail | 2022 |
| Ouston Farms Limited | Agriculture & fishing | 1977 |
| Outcome Engineering Ltd | Professional & scientific | 2024 |
| Outsourcing Procurement Limited | Administrative & support | 2014 |
| Oval Energy Limited | Other services | 2018 |
| Overafter Limited | Energy | 1996 |
| Overheads UK Limited | Professional & scientific | 2004 |
| Owen James Digital Ltd | Information & communication | 2020 |
| Owen Shaw Awards Ltd | Wholesale & retail | 2026 |
| Owens Group Limited | Administrative & support | 2018 |
| Oxford Aero Limited | Wholesale & retail | 2023 |
| Oxford MOT & Service Centre Ltd | Wholesale & retail | 2018 |
| Oxford Street Car Sales Ltd | Wholesale & retail | 2023 |
| Oxley Renwick 1 Limited | Administrative & support | 2014 |
| Oxley Renwick 2 Limited | Administrative & support | 2014 |
| Oyster North East Ltd | Hospitality | 2017 |
| Ozmo Limited | Health & social care | 2022 |
| Ozone Healthcare Limited | Other services | 2024 |
| Ozpa Property Services Ltd | Real estate | 2023 |
| P & a Kennedy Limited | Professional & scientific | 2019 |
| P a Moody Recycling Limited | Water & waste | 2005 |
| P Dixon Ltd | Transport & logistics | 2016 |
| P Helena Ltd | Hospitality | 2025 |
| P I C Investments Co. Limited | Households | 2018 |
| P Miller Plumbing Heating and Inspection Services Ltd | Administrative & support | 2016 |
| P North Developments Ltd | Construction | 2013 |
| P North Group Limited | Real estate | 2009 |
| P S Wilson Properties Ltd | Real estate | 2025 |
| P Sweeney Engineering Services Limited | Administrative & support | 2013 |
| P&Ah Property Ltd | Real estate | 2020 |
| P. S. Pendergood Limited | Wholesale & retail | 2008 |
| P.F.D. Surgical Ltd | Health & social care | 2024 |
| P.I.G. Estates Limited | Real estate | 2002 |
| P2L Properties Limited | Real estate | 2006 |