| On-Line Data | Attica |
| ON.marketing | Attica |
| Onspot | Attica |
| Onyro.com | Attica |
| OpenCofee.gr | |
| Opencoffeee.gr | Attica |
| Openfund | Attica |
| OpenThessaloniki | Central Macedonia |
| OPT Hellas SA | Attica |
| Optimum S.A. | Attica |
| Optisoft | Attica |
| Optiva LCS Hellas | |
| Optronics | Attica |
| Oracle Greece | Attica |
| Orgin Studios | Attica |
| OTEGLOBE S.A. | Attica |
| Oyster Hellas | |
| P. Moschopoulos & Co | Central Macedonia |
| PA Web Gate | Attica |
| Panasoft S.A. | |
| Panou | Attica |
| Panteion University of Social and Political Sciences | |
| Pantso | Attica |
| Papaki | Crete |
| Paramarketing.gr | Attica |
| Patlis.com | Attica |
| PC-Shop | Attica |
| PC2Help | Attica |
| Petros Dimitriadis | Attica |
| PG-Software | Attica |
| Phaistos Networks | Attica |
| Pharmathen S.A. | Attica |
| Phasmatic | Ionian Islands |
| PI-Systems International | Attica |
| Piraeus Bank | Attica |
| Plaisio Computers | Attica |
| Planetek Hellas | |
| Play247 | Attica |
| Plegma Labs S.A. | |
| Plushost.gr | Attica |
| Point Blank | Central Macedonia |
| Pointer O.E. | Attica |
| Porcupine Colors | Central Macedonia |
| Positron S.A. | Attica |
| Power Team | |
| PowerX.tech | |
| Presence on the web | Attica |
| Privenet | Attica |
| ProCredit Bank Greece | |
| Profile Software | Attica |