| NuFlare Technology Co., Ltd | Kanagawa |
| Nulab | Fukuoka |
| NYK Business Systems Co., Ltd | Tokyo |
| Oak Capital Corporation | Tokyo |
| OAK Co., Ltd | Kanagawa |
| Obihiro University | |
| Obunsha Ventures, Inc. | |
| OCC Corporation | Okinawa |
| Oceanize, Inc. | |
| Ochanoko-net, Inc. | Tokyo |
| Ochanomiz University | |
| Ochanomizu University - Mathematics | Tokyo |
| Office R3 | Osaka |
| Office System Integration Co., Ltd | Tokyo |
| Og, Inc. | Tokyo |
| Ogaki Women's College | |
| OGN | Hokkaido |
| Ohara Memorial Institute for Science of Labour | |
| OHARA Pharmaceutical Co., Ltd | |
| Ohara, Inc. | |
| Ohka Gakuen University | |
| Ohtsuki City College | |
| Ohu University | |
| Oink Games, Inc. | |
| Oita University | |
| Oita University of Nursing and Health Sciences | |
| Oita Venture Capital Co., Ltd | |
| Okano Cable Co., Ltd | |
| Okayama Network, Inc. | Okayama |
| Okayama Shoka University | |
| Okayama University | |
| Okayama University - Department of Mathematics | Okayama |
| Okayama University of Science | |
| Okazaki Women's University | |
| Oki Electric Cable Co., Ltd | |
| Oki Electric Industry Co., Ltd | Tokyo |
| OKI IDS Co., Ltd | |
| Okinawa Cable Network, Inc. | Okinawa |
| Okinawa Christian Institute. | |
| Okinawa Development Finance Corporation | |
| Okinawa FreeBSD Users Group | Okinawa |
| Okinawa Institute of Science and Technology | |
| Okinawa International University | |
| Okinawa Prefectural College of Nursing | |
| Okinawa Prefectural University | |
| Okinawa Prefectural University of Arts | |
| Okinawa University | |
| Okura Information System | Tokyo |
| Olinet | Tokyo |
| OLIOSPEC | Tokyo |