| Orca Computing | |
| Orcadia Design Ltd | Orkney |
| Orchard Rock Training Ltd | |
| Orchard Therapeutics plc | |
| Orchardweb | Carmarthenshire |
| Orchid Box Ltd | |
| Orchid Data Systems Ltd | Northamptonshire |
| ORCHID Information Systems Ltd | Hertfordshire |
| Orchid Software | Tyne and Wear |
| Orckid Design and Marketing Ltd | Surrey |
| Orckit Communications Ltd | West Sussex |
| Ord Technology Ltd | |
| OrderCast Ltd | |
| Orderswift | |
| OrderWise | Lincolnshire |
| OrderWork Ltd | Bedfordshire |
| Ordinary Media | |
| Ordo | |
| Orebix Digital | Cheshire |
| Oregan Networks Ltd | Greater London |
| Oregen Training | East Sussex |
| OrganOx Ltd | |
| Ori Biotech Ltd | |
| Ori Industries | |
| Origin Design and Marketing Ltd | Buckinghamshire |
| Origin Markets Origin Primary Ltd | |
| Origin Sciences | |
| Origin Software Solutions | |
| Origin Storage | Hampshire |
| Original Business IT Solutions Ltd | |
| Original Software | |
| OriginalScan Computer Services | |
| Oriium Consulting Ltd | West Yorkshire |
| Oriole Networks | |
| Orion Consulting Ltd | Rutland |
| Orion Pharma (UK) Ltd | |
| Orion Practice Management Systems Ltd | |
| Orion247 | |
| Orka Technology Group | |
| Orka Technology Group Ltd | |
| Orkis Media | Leicestershire |
| Orlo | |
| Ormsby Street | |
| Orpheus Internet Services | Essex |
| Orris Digital | Hertfordshire |
| Ortecha Ltd | |
| Orthogonality Ltd | Cambridgeshire |
| OrthoSon Ltd | |
| Orthox Ltd | |
| Ortiga | West Midlands |