The object is clearly the computer. The operating system operates the computer.
The indirect subject is clear. The operating system operates on behalf of its human user community.
The direct subject is less clear. Generally, the human users interact with applications programs of one kind or another, and it is these programs on behalf of which the operating system operates.
This question was posed to the class, and the following responses were discussed:
With the advent of personal computers, there was even talk about the irrelevance of operating systems, since with only one user, it was easy to imagine that there were no resource contention problems remaining.