| OM Digital Solutions Corporation | Tokyo |
| OMRON Corporation | Kyoto |
| OncoTherapy Science, Inc. | |
| One Day System Co., Ltd | Aichi |
| One Terrace Co., Ltd | |
| OneAI Co., Ltd | Fukuoka |
| Onebox Co., Ltd | Tokyo |
| Onetap Co., Ltd | Tokyo |
| Ono Pharmaceutical Co., Ltd | |
| Onodera GT Pharma | |
| Onomichi City University | |
| Open Network Lab | Tokyo |
| Open Source Robotics Foundation | |
| Open University of Japan | |
| Openlogi Co., Ltd | Tokyo |
| OpenLogi, Inc. | |
| OpenStage Co., Ltd | Tokyo |
| Operations Research Society of Japan | Tokyo |
| Optex Co., Ltd | Shiga |
| Optimind Co., Ltd | Aichi |
| Optorun Co., Ltd | Saitama |
| Opus Studio, Inc. | Tokyo |
| Oracle Japan | Tokyo |
| Orange Cube Co., Ltd | Tokyo |
| Orange Internet Service | Tokyo |
| Orangesoft, Inc. | Tokyo |
| Orario Co., Ltd | Tokyo |
| Orb Co., Ltd | Tokyo |
| ORBITAL Engineering Co., Ltd | Kanagawa |
| ORC Manufacturing Co., Ltd | Tokyo |
| Organo Corporation | Tokyo |
| Oriental Motor Co., Ltd | |
| Original Soft Co., Ltd | Tokyo |
| Orio Aishin Group | |
| ORIX Corporation | |
| Osaka Aoyama University | |
| Osaka Christian College | |
| Osaka City University | |
| Osaka Dental University | |
| Osaka Electro-Communication University | |
| Osaka Gakuin University | |
| Osaka Institute of Technology | |
| Osaka International Education Institution | |
| Osaka Jogakuin University | |
| Osaka Kawasaki Rehabilitation University | |
| Osaka Kyoiku University | |
| Osaka Ohtani University | |
| Osaka Prefecture University | |
| Osaka Research Institute of Industrial Science and Technology | |
| Osaka Sangyo University | |