| Onit Web Solutions Ltd | Tyne and Wear |
| Online Backup from Backup Technology Ltd | |
| Online Design | Derbyshire |
| Online Growth Guru Ltd | |
| Online Integrity | |
| Online Marketing Surgery | |
| Online Media Direct | |
| Online Practice | West Midlands |
| Online Support Computing Ltd | Greater London |
| Online Systems (UK) | Nottinghamshire |
| Online Systems Ltd | Cumbria |
| OnlineMarketing.org.uk | |
| OnlineStore | Tyne and Wear |
| OnLiTECH Ltd | Berkshire |
| Onoco | Greater London |
| Onport | |
| ONQOR Group | |
| Onside Creative | |
| Onsite Pro | |
| Onstage | |
| Onto | |
| Onvi Tech Ltd | |
| Onwatch CCTV Ltd | Kent |
| Onyx Scientific Ltd | |
| Onyx Tech Solutions | |
| oomph! | Angus |
| Ooodles Finance Ltd | |
| Opace Ltd | West Midlands |
| Opace Ltd | |
| OpalTec International Ltd | Bristol |
| Open Automation UK | |
| Open Connections Solutions Ltd | Bristol |
| Open Cosmos Ltd | |
| Open Credo Ltd | |
| Open Data Handbook | Cambridgeshire |
| Open Data Manchester | |
| Open Data Services Co-operative Ltd | |
| Open eLMS | Greater London |
| Open GI | Worcestershire |
| Open Government Data | Cambridgeshire |
| Open Grid Systems Ltd | Glasgow |
| Open Health Care UK | |
| Open Knowledge Foundation | |
| Open Link Systems | |
| Open Network Associates Ltd | Bristol |
| Open Planet Software | |
| Open Tech Calendar | |
| Open Trade Technology Ltd | |
| Open University | |
| Open University - School of Mathematics and Statistics | Buckinghamshire |