| On Budget and Time Ltd | |
| On Device Research Ltd | |
| On Screen | West Yorkshire |
| On SEO | |
| On The Spot Development | |
| On The Spot Internet | Glasgow |
| On2net (UK) Ltd | Staffordshire |
| OnBrand | |
| OnCare | |
| Once Upon A Time Marketing Ltd | |
| Oncimmune | |
| OncoLytika | |
| Ondato | |
| One AV | Worcestershire |
| One Beyond Ltd | |
| One Hat Design Ltd | |
| One HealthTech | |
| One IT Services and Solutions | |
| One Line | |
| One Page Websites | |
| One Step Design | Swansea |
| One Tribe Global Ltd | |
| One Utility Bill | |
| One Wavelength Ltd | Bedfordshire |
| One World Connect Ltd | |
| One2create | Hampshire |
| one2one Digital Ltd | |
| OneAgency | Norfolk |
| Onebite Ltd | |
| OneClick Media Services | Greater London |
| Onecom Ltd | Hampshire |
| oneHR | |
| OneID Ltd | |
| Onekcode | |
| Onepoint Consulting Ltd | |
| OnePoint Systems Ltd | |
| OnePound WebHosting | Hampshire |
| Ones and Zeros | Leicestershire |
| Onesource IT | Leicestershire |
| Onestop IT Solutions | Edinburgh |
| Onestop Webshop | Norfolk |
| Onesys | |
| OneUp Sales | |
| OneUpOneDown | Hertfordshire |
| OneWeb | |
| ONFAB Ltd | |
| Onfido | Greater London |
| OnFire Agency | |
| Onimpact | Berkshire |
| OnIT Services Ltd | Buckinghamshire |