Since the emergence of OSS, it has brought many benefits to computer systems. However, OSS has changed traditional software lifecycle management. It is pointed out that there is no clear distinction between developers and users and that users are continuously involved in OSS development. Therefore, OSS must be taken care of also in the sense of software maintenance. In this paper, we propose two measures, "delay front" and "skip interval" for assessing OSS maintenance. For a given benchmark suite, activities of IBM and Apple, two major system vendors supporting OSS, are assessed. Furthermore, OSS support activities by major computer centers for shared use are also assessed. By the OSS feature that the development processes and upgrade dates are open to all users, these measures can be explictly calculated, and are useful for calling attentions to the status of OSS support. By checking them, we can evaluate the efforts made by system vendors and computer centers.