[LoC: Prologue, The First Message, 59-61]
Demandred [WH: 13, Wonderful News, 314-5]
[WH: 35, With the Choedan Kal, 645]
First of all, Ishamael was far too unstable and insane (I believe) to such a calm and somewhat accepting born-again Chosen. Nothing in his bio in "The World of Robert Jordan's Wheel of Time" matches with the clues given so far, so I will rule him out strictly on the basis of a lack of evidence. (That insanity bit does have something to do with it...)
Sammael can be brought into question, primarily because in the Age of Legends, he "was ruggedly handsome. His compact physique made him seem larger than he actually was. When compared to other men, he was only of average height (TWoRJWoT, The Dark One and the Male Forsaken, 53)." (Refer to the "clue" above about Osan'gar and being average.) However, nothing has been written of his age when awakened, so one would assumed that he was not as old as Aginor or Balthamel. Furthermore, there is not proof that Sammael is even dead. True, Rand did see him vanish into the mist Mashadar in Shadar Logoth, but is he really dead? I would tend to believe not, considering that he was still alive after the first appearance of Osan'gar and Aran'gar. (But that does not rule out Cyndane...)
Asmodean was described as "a dark-eyed, dark-haird, handsome man (TWoRJWoT, 59)," and he also had his brushes with greatness, yet fell short. As a composer, he "never rose to the exalted heights that many had foretold, and was never ranked among the great composers of the Age (TWoRJWoT, 59)." However, that is all the information in his history that links him to Osan'gar, so again, I will rule him out for lack of evidence.
According to "Lord of Chaos," page 59, Osan'gar noted that his faced was "[not] young, but younger than the faced he had worn on first waking from the long sleep." In "The World of Robert Jordan's Wheel of Time," the entry about Aginor describes him as "trapped in a body impossibly aged and withered from the grinding of the Wheel over the long years of his captivity. Because of the immortality granted by the Dark Lord, he was alived, but perhap because of the effects of his imprisonment, his body had suffered the ravages of time (TWoRJWoT, 53)." However, Balthamel was trapped closer to the surface of the seal than Aginor, and like Aginor, Balthamel "also suffered from the passage of time, but to a much greater extent. His soul and spirit were still vital, but his once handsome body had rotted to the point that he could no longer bear to have it seen (TWoRJWoT, The Dark One and the Male Forsaken, 54)." In addition, Balthamel was also "A good-looking man who enjoyed the company of women and was very popular with them... he spent a great deal of time in establishments that today would be called taverns of the lowest sort (TWoRJWoT, 53)." He was drawn to serve the Dark One because of "the promise of immortality. To live forever and never age; as simple as that (TWoRJWoT, 53)." This could reflect back to the note from "Lord of Chaos" which says "he had always hated ordinary (LoC, 59)." I am left with the decision between Balthamel and Aginor. The only strong point that I can use to diffuse one is this: Balthamel was "a good looking man" whose "once handsome body" had rotted away. That doesn't sound average to me. So... who was Osan'gar? I put my money on Aginor.Well, as suspected, Osan’gar was revealed to be the reincarnatioin of Aginor in Winter’s Heart. Okay, so not 100%, but the extra evidence, when compared to the historical info for Balthamel really cinches it.
It was also revealed that Osan’gar has been hiding out, disguised as the Asha’man named Corlan Dashiva. He was somewhat behind the assassination attempt on Rand at the end of The Path of Daggers, and we can assume that he was involved with Mazrim Taim somehow... But we’ll have to wait and see on that one.
But, poor Osan’gar was killed at the end of Winter’s Heart by a Darkfriend, no less. One Black Sister, Elza Penfell Sedai, used (what I believe to be) balefire on the guy, turning the hill that he was hiding behind into a literal memory. Nothing but an empty space existed at the end, and even Cadsuane Sedai mentioned that all they got out of the final battle was “one dead renegade.”