| Once Upon a Time Children's Day Nursery Limited | Education | 2016 |
| One Executive Emea Ltd | Administrative & support | 2020 |
| One Key Group Limited | Professional & scientific | 2025 |
| One Laser Ltd | Administrative & support | 2022 |
| One OFF Machining Ltd | Manufacturing | 2022 |
| One Serenity Ltd | Wholesale & retail | 2006 |
| One Umbrella Maintenance Ltd | Administrative & support | 2023 |
| Oneciti Asset Management Ltd | Real estate | 2023 |
| Onley Grounds Equestrian Complex Limited | Arts & recreation | 2006 |
| Onlymot (WKP) Limited | Professional & scientific | 2018 |
| Onlymot BTN Limited | Professional & scientific | 2012 |
| Onlymot GTM Limited | Professional & scientific | 2012 |
| Onlymot(Ctn) Limited | Professional & scientific | 2012 |
| Onlymot(Lnc) Limited | Professional & scientific | 2012 |
| Onlymot(Mfd) Limited | Professional & scientific | 2012 |
| Onlymot(Nwk) Limited | Professional & scientific | 2012 |
| Onlymot(Swl) Limited | Professional & scientific | 2025 |
| Onntime Fleet Express Ltd | Transport & logistics | 2025 |
| Onside Coaching Coventry and East Warwickshire (Childcare and PE) Ltd | Education | 2018 |
| Ontime Trans Ltd | Transport & logistics | 2019 |
| Opal Advisory Limited | Professional & scientific | 2024 |
| Open Doors Lettings Ltd | Real estate | 2025 |
| Open Doors Logistics Ltd | Transport & logistics | 2025 |
| Open Energy Ltd | Construction | 2026 |
| Open EYE 24/7 Limited | Information & communication | 2022 |
| Open Sky Self Storage LLP | | 2025 |
| Open Sky Ventures Limited | Wholesale & retail | 2020 |
| Opswell Core Ltd | Other services | 2025 |
| Opticax Ltd | Information & communication | 2021 |
| Optimised Operations Limited | Professional & scientific | 2001 |
| Optimum Link Engineering Ltd | Professional & scientific | 2025 |
| Optimum TA Ltd | Administrative & support | 2024 |
| Opulence Enterprises Ltd | Wholesale & retail | 2025 |
| Opusol Limited | Real estate | 2014 |
| Orange Accountancy Services Ltd | Professional & scientific | 2021 |
| Orange Consultancy Ltd | Professional & scientific | 2025 |
| Orangehomesuk Ltd | Construction | 2026 |
| Oras Marketing Limited | Wholesale & retail | 2020 |
| ORB Marketing Services Limited | Professional & scientific | 2004 |
| Orbis Terrarum Ltd | Professional & scientific | 2025 |
| Orbital Welding Supplies Limited | Manufacturing | 2015 |
| Orchard Acuity Limited | Professional & scientific | 2017 |
| Orchid Purple Limited | Health & social care | 2022 |
| ORG Design Consulting Ltd | Professional & scientific | 2016 |
| Origin LDN Limited | Professional & scientific | 2020 |
| Originate Planning Limited | Professional & scientific | 2020 |
| Orionrise Consulting Ltd | Professional & scientific | 2023 |
| Orite Drive Limited | Transport & logistics | 2015 |
| ORO X Spirits UK Limited | Manufacturing | 2023 |
| Orscheln Europe | Administrative & support | 1998 |
| Orthotika Ltd | Extraterritorial | 2019 |
| Osbi Real Estate Ltd | Real estate | 2023 |
| Osborne Sargent Ltd | Real estate | 2024 |
| Osbourne Limited | Financial services | 2020 |
| Oscars Hair Design Ltd | Other services | 2008 |
| Osgodby Solar Limited | Energy | 2021 |
| Osiatynski Bespoke Education Limited | Education | 2020 |
| Osprey Air Services Limited | Transport & logistics | 1984 |
| Otbc Properties Ltd | Real estate | 2019 |
| OTR Plumbing & Heating Ltd | Construction | 2024 |
| OTT365 Limited | Transport & logistics | 2018 |
| OUR 4 Walls Ltd | Wholesale & retail | 2023 |
| Oureast Innovation Consulting Company Limited | Administrative & support | 2008 |
| Outcomes Consultancy Ltd | Wholesale & retail | 2025 |
| Outpace Group Ltd | Information & communication | 2019 |
| Outright Acquisition Group Ltd | Information & communication | 2023 |
| Outright Acquisition Ltd | Construction | 2019 |
| Outsource Essentials Limited | Administrative & support | 2025 |
| Overhead Power Limited | Professional & scientific | 2015 |
| Overland and Sea the Sameday Company Limited | Transport & logistics | 2007 |
| Overland Real Estate Partners LLP | | 2026 |
| Ovo&B Ltd | Transport & logistics | 2017 |
| Owen Froebel Limited | Real estate | 2022 |
| Owning Safety Limited | Professional & scientific | 2023 |
| Oxford Bridge Academy Limited | Education | 2026 |
| Oxford Consultants Ltd. | Professional & scientific | 2023 |
| Oxford Motor Club Limited | Other services | 2012 |
| Oxyjet UK Ltd | Wholesale & retail | 2024 |
| Oyetoks Enterprises Ltd | Health & social care | 2025 |
| Oyster Bay Holdings Limited | Real estate | 2020 |
| OZI Communication Limited | Information & communication | 2020 |
| Oznek Technologies Ltd | Information & communication | 2019 |
| P & H Builders Ltd | Construction | 2015 |
| P Badwal Properties Limited | Hospitality | 2013 |
| P D Supplies Limited | Wholesale & retail | 1996 |
| P H M Contract Services Limited | Transport & logistics | 2019 |
| P K Siva Investment Ltd | Real estate | 2023 |
| P K Siva Limited | Wholesale & retail | 2018 |
| P&C Digital Ltd | Professional & scientific | 2026 |
| P&M Labour Limited | Construction | 2016 |
| P-Brown Ltd | Health & social care | 2025 |
| P. Morgan Limited | Health & social care | 2003 |
| P.Burton Limited | Construction | 2016 |
| P.C Road Delivery Ltd | Transport & logistics | 2026 |
| P.Lorinc Limited | Transport & logistics | 2014 |
| P1 Smart Repairs Limited | Wholesale & retail | 2017 |
| P2MS Precision & Predictive Maintenance Services Ltd | Professional & scientific | 2025 |
| P4TKD Ltd | Arts & recreation | 2006 |
| P8D Ltd | Transport & logistics | 2013 |
| PA55 RBB Ltd | Education | 2026 |