Chosen one is the person that is born or starts the game as the one that whatever is happening, it is about him directly and the ones behind it know and care what happens to him from the start. Example Baldur's Gate 1, MotB.
Unique person story is not directly connected to you but your uniqueness gets you into the middle of it. Example, Kotor 1 and 2 (well Kotor 2 could be also chosen one story if we take into account you are directly being groomed the whole game but that story is kind of messy and unfinished)
NWN OC, I don't remember too well if something happens to you directly at start. I don't think so. That story is not Chosen One or Unique. You are not directly connected to main story in any special way except that you have responsibility as a Lord to help the people there. FO:NV I have not played enough to know for sure, but if I got it right your character survives by fluke and is in no way chosen one or unique. Fallout 1 and 2 would be Unique storyline.
EDIT: OK, Nonek just said you get wounded by the Shard while young? I really don't remember that part but if it is true that would make NWN OC the Unique storyline.