| Open University - Science, Technology, Engineering and Mathematics | Buckinghamshire |
| Openbrolly | |
| Opencast | |
| OpenCell Labs | |
| OpenCorporates Ltd | |
| OpenCRM | North Yorkshire |
| Openda Ltd | Wiltshire |
| Opendesk | |
| OpenDialog AI Ltd | |
| OpenFaaS Ltd | |
| OpenGamma Ltd | |
| OpenGlobal | Gloucestershire |
| OpenPlay | |
| OpenRent Ltd | |
| Opensignal Ltd | |
| Opensoft Systems Ltd | North Yorkshire |
| OpenUK | Merseyside |
| Openxtra | West Yorkshire |
| Operational Accounting Systems Ltd | |
| Operational Solutions Ltd | |
| OPIFER Ventures | |
| Opinew | |
| Opinium | |
| Opinly Ltd | |
| Opinov8 | |
| Opiyo.Digital | Gloucestershire |
| Oporteo | |
| Opphive | |
| Oppilotech Ltd | |
| Opposable Games Ltd | |
| Opsly Cloud Ltd | |
| Opsview Ltd | Berkshire |
| Optalitix | |
| Optalsys Ltd | |
| OptaNet Ltd | |
| Optasia Medical | Greater Manchester |
| Optech Fibres Ltd | |
| Optellum Ltd | |
| OptiBiotix Health plc | |
| Optibrium | |
| Optical Technology Training Ltd | North Yorkshire |
| Optics.org | |
| Opticus | West Dunbartonshire |
| Optify Mediaforce Ltd | Greater London |
| Optillion Ltd | Nottinghamshire |
| Optima Connections Ltd | Buckinghamshire |
| Optima Energy | |
| Optima HR | |
| Optima Legal Services Ltd | |
| Optima Systems Ltd | West Sussex |