| OncoSil Medical Ltd | New South Wales |
| Ondetto | |
| One AML Review Pty Ltd | New South Wales |
| ONE BCG Pty Ltd | New South Wales |
| One Click Life | |
| One IT Services | |
| One Stop Media | |
| One Stop Warehouse Pty Ltd | |
| One Touch Web Designs | |
| One World Media | |
| One10 | Victoria |
| Onehalf Offshore Business Solutions | |
| Oneit.com.au | Western Australia |
| OneMD | New South Wales |
| OneVentures Pty Ltd | New South Wales |
| Online Advantage | New South Wales |
| Online Marketing Gurus | New South Wales |
| Online Marketing Whiz | |
| Online Presence | |
| Online Solution Experts | |
| Online Visions | Queensland |
| OnlineConsulting.com.au Pty Ltd | New South Wales |
| OnMyWeb | New South Wales |
| Onpoint Creations | New South Wales |
| Onsavii | |
| OntheNet | Queensland |
| Ontologics Pty Ltd | Queensland |
| Op Central | Victoria |
| Opacity Design | New South Wales |
| OPC | |
| Open Colleges Pty Ltd | |
| Open Copy | Western Australia |
| Open Money Group Pty Ltd | New South Wales |
| Open Source Industry Australia Ltd | Australian Capital Territory |
| Open Spatial Pty Ltd | New South Wales |
| Open Systems Technology | South Australia |
| OpenLearning Ltd | |
| Openmarkets Australia Ltd | New South Wales |
| Openmesh | New South Wales |
| OpenSolar Pty Ltd | New South Wales |
| OpenSpatial | New South Wales |
| OpenTable | |
| Optain | |
| Opthea | Victoria |
| Opticalfibre | Queensland |
| Optima Technology | Victoria |
| Optimal Virtual Employee | |
| Optimation Software Engineering Pty Ltd | Victoria |
| Optimax | |
| Optimise Online | |