In fact, PlayOneShot doesn't affect isPlaying. It starts the sound and gets ready to another one, even if the first is still playing. Play can start a new sound too, but the prior sound is stopped if still playing.
You can work around this by emulating a version of PlayOneShot based on Play, like this:

var sound1:AudioClip;
var sound2:AudioClip;
var sound3:AudioClip;
 .
function playSound(sound:AudioClip, vol:float){
  audio.clip = sound;
  audio.volume = vol;
  audio.Play();
}
 .
// play a sound this way:
  playSound(sound2,0.8);
 .

I worked around this by writing my own isPlaying() method while wrapping a class around my audioclips. Hope it helps!

private float startTime;

private void PlayOneShot(AudioClip clip, float volume)
{
    audioSource.PlayOneShot(clip, volume);
    startTime = Time.time;
}

public bool isPlaying()
{
    if((Time.time - startTime) >= clip.length)
       return false;
    return true;
}

I don't have an answer per se, but I can confirm the behavior you describe.

I'm tempted to say it's a bug, and might be worth reporting as such (it seems that the 'is playing' property should be useable even when the sound was played using PlayOneShot()). However, it may be the intended behavior (I'm not sure).

[Edit: Based on what aldonaletto says, I guess it's the intended behavior.]

[Edit 2: Just tried it out, and it looks like it is indeed the correct/intended behavior. I have to admit I didn't know PlayOneShot() functioned that way (the docs don't mention it), but I suppose it makes sense that it does.]