| Ocean Website Design | |
| OceanicDesign.com | Cambridgeshire |
| Oceanology International | |
| Ochre Bio | |
| Ochresoft Technologies Ltd | |
| OCM Business Systems Ltd | Worcestershire |
| OCM Communications Ltd | Warwickshire |
| OCM Digital Media | |
| OCP | |
| OCPR Ltd | |
| OCS Consulting | Hertfordshire |
| OCS Studios | |
| OctaDigital Web Design | |
| OctaGenix Ltd | |
| Octagon IT | |
| Octagon Technology Ltd | |
| Octal Info Solution Ltd | |
| Octane Interactive Ltd | South Yorkshire |
| Octatude | |
| Octopus Energy Ltd | |
| Octopus Energy Ltd | |
| Octopus Systems | Suffolk |
| Octopus Ventures | |
| Octree Ltd | |
| Ocula Technologies | |
| Oculo Technologies Ltd | Greater London |
| Ocutrx Technologies International Ltd | Greater London |
| Oddslife Ltd | |
| Oddsoft Ltd | Suffolk |
| Odiin Ltd | |
| Odline | Surrey |
| Odore | Greater London |
| Odro Ltd | |
| Odyssey New Media | West Midlands |
| Odyssey Ventures | |
| Ofcom | Greater London |
| OFEC Consulting Ltd | Oxfordshire |
| Off The Peg Design Ltd | |
| Off-Piste Digital Ltd | Lancashire |
| Office Automation Ltd | Greater London |
| Office Freedom | |
| Office King | Lancashire |
| Office Machine Maintenance | Northamptonshire |
| Office Tech Suite | |
| OfficeLabs | |
| Offices.co | |
| Offshore Electronics | Channel Islands |
| Offshore Internet Services Ltd | Channel Islands |
| Offtek Ltd | West Midlands |
| OFIXU Ltd | |