| Optimal Industrial Automation Ltd | Bristol |
| Optimal Labs Ltd | |
| Optimalworks Ltd | Devon |
| Optimise Consultancy Ltd | |
| Optimise Solutions Ltd | Edinburgh |
| Optimise Web | |
| Optimise.London | Greater London |
| Optimize | |
| Optimo | |
| Optimum Click Ltd | |
| Optimum SME Finance Ltd | |
| Optimumcut Software | Wiltshire |
| OptimumProgram | |
| Optimus Group Ltd | Surrey |
| Optinet | |
| Optio | |
| Option Systems Ltd | Leicestershire |
| Optisoft Ltd | North Yorkshire |
| Optiva Ltd | |
| Optix Solutions Ltd | |
| Optoelectronic Manufacturing Corporation (UK) Ltd | Cornwall |
| Optos | Fife |
| OptoSci Ltd | Glasgow |
| Optus Services Ltd | Berkshire |
| Opus Business Systems Ltd | |
| Opus Software Ltd | Lincolnshire |
| Oracle Corporation UK Ltd | Greater London |
| Oranda Design | Bristol |
| Orange Crush Digital | West Yorkshire |
| Orange Tree Digital | Derbyshire |
| Orange Tree Technologies Ltd | Oxfordshire |
| Orangeleaf Systems | Shropshire |
| Orantec | Cardiff |
| Orate Ltd | Suffolk |
| Orbex | Moray Council |
| Orbiss Ltd | Cornwall |
| Orbit Discovery | |
| Orbit Multimedia | Northamptonshire |
| Orbit Office Supplies | |
| Orbit Online | |
| Orbit Services | |
| Orbit29 | |
| Orbital | |
| Orbital Design Ltd | |
| Orbital Net | Kent |
| Orbital Witness Ltd | |
| Orbital10 | |
| Orbital10 Ltd | |
| ORBN Ltd | |
| Orca AI Ltd | Greater London |