Ever since reading the theory that Dashiva is Osan'gar, I have thought and thought about it, and come to the conclusion that it does fit quite nicely.
First of all, the names Aran'gar and Osan'gar are the left and right hand daggers in a sport from the Age of Legends. Most people know this, and the fact that both participants usually die because of the poison on the blades... Left and right are mirror images of each other... (I am a mirror twin, soI understand this well.) We know Halima is Aran'gar, and is in the "True" or "Little" White Tower... The mirroring would imply that Osan'gar is, indeed, at the Black Tower... Remember Yin and Yang...
I don't have the evidence in front of me to support the face touching information, but other posts discuss this much better.
Next, I refer to the Chapter "A Time for Iron" in The Path of Daggers. On p531 of the paperback, things come to a head. This is the encounter where Dashiva lectures Rand about the "weirdness" on saidin, and the dangers inherent in the situation.
Note Lews Therin's reaction to Dashiva weaving the ward. He doesn't even snarl, he just grabs for the Power. Is it possible that he "knows" Aginor's method of weaving? As in, he recognized Aginor through Aginor's touch (???) or style of weaving? Just grasping here, but it might be important... Same thing for Taim and Demandred, by the way... sorry, wotmania...
Also note the point where Rand mentions that he has channeled longer than Dashiva. Dashiva almost turns to ice, and snaps that he is "well aware" of how long Rand has been channeling. Given that Rand's first MAJOR act of channeling was to face off against Aginor at the Eye of the World, that might be the rub...
The reason for the subject is the line: "I don't like strange applied to saidin, and I don't want to die or... or be burned out because you're blind!" (The Path of Daggers, p531)
The interesting thing is the ellipsis. Why would he hesitate? It could be just emotion, or it could be emotion almost made him slip, and say "severed," or some other Age of Legends term for burning out, and reveal who he REALLY was...
That's my thoughts on the subject... I was also reading around, and I noticed one possible hole. Is there any mention anywhere on how good a healer Aginor was? He was a biological scientist, so you think he would be pretty good, but Dashiva can't Heal anything... But he may just be hiding his ability.
wotmania says: I really liked this theory until you threw in that comment about Taimandred.
Seriously, there are some very good points in here - I particularly like the comment that Dashiva is "well aware" of how long Rand has been channeling. I can see where Dashiva might take that the wrong way if he really is Osan'gar - Aginor just might be a bit bitter about being killed approximately four seconds after leaving the Bore...
Lord Avatar Darkstorm 
King of the Morning 
Prince of the Day 
He Who Rules the Night

Karlaan 
Hindsight