Ok look.... One Winged Angel is the single most mixed, remixed, rerecorded, revamped, and re-anything-ed track of anything even remotely related to final fantasy...
You are looking for a specific version, by giving only the basic title and where you found it once...
I suggest you go on playlist[dot]com and search for 'one winged angel' and start listening to every single file it finds until you get the right one...
iTunes is for the unenlightened... like WoW... their populartiy is offset by nothing else but their stupidity...
There are the FFVII version, which was a PSX music file with vague clarity but catchy nonetheless...
There is the redacted AC prototype version, it was a full orchestral remake of the PSX version, but, it was scrapped for the metal version...
The metal version used in AC... it was performed by The Black Mages (under their original Japanese name)
The KH version
The KHII version
The innumerable shitty midi versions
The Piano version
and so on