I know there have been bits and pieces about this subject. It is now evident that Aginor is Osangar is Dashiva. I provide a comprehensive theory on the subject. I apologize that it is quite long, I needed to get in most of the relevant info with cites.

The Evidence (I add the emphasis through) Also, I make comments after most of the passages, I cant color them so try and understand which words are my comments and which are the quotes":

ISSUE: WHO IS DASHIVA?
FACTS:

The following is Osangar upon getting his new body:

LOC (Soft) p. 70 - "... Still disbelief. It reflected his own face, too, and that interested him far more than the woman. He could not resist touching his nose and mouth and cheeks for the hundreth time to make sure they were real. Not young, but younger than the face he had worn on first waking from the long sleep, with endless nightmares. An ordinary face, and he had always hated being ordinary. He recognized the sound in his throat as a budding laugh, a giggle, and stifled it. He was not mad. Despite everything, he was not that."

p. 71 "... He had never liked Halfmen. He had helped make the Trollocs, blending animal and human stock - he was proud of that, of the skill involved, the difficulty . . ."
This demonstrates that we are dealing with Aginor here who created the Trollocs.

p. 72 " . . . He backed away with raised hands . .. .He had always prided himself on his sense of humor, but this was rich." Dashiva has these reactions, see below

". . . He would grovel before a worm to avoid that(death) again . . . " - He would grovel before LTT too
Shaidar Haran: "If you fail . . . But you will not fail, willyou?"

The Halfman Did smile then. It was like seeing death smile." - This is why he freaks about Rand being ill in COS, he cant failthe DO again
The following is when Rand and Taim discuss an honor guard for the Dragon Reborn in the aftermath of Dumai Wells: The rest of the citations are from COS (Hardcover)

P. 83 . . . "My Lord Dragon," Taim broke in, "you need morethan a few Ashaman around you." (Perrin then analyzes their scent and Taim tells Rand he needs Ashaman not just soldiers or dedicated) . . .

P. 84 . . ."One," Rand cut in. "And I will choose." (Taim reacts) "Him." This time, he Seemed surprised to find he was pointing directly at a man in his middle years Sitting atop an upturned cask on the other side of the wagon circle, paying no Attention to the gathering around Rand. Instead, elbow on his knee and chin Propped on his hand, he was frowning at the aes sedai prisoners. . . (Rand asks Taim his name) "Corlan Dashiva. From a farm in theBlack hills." Middle age matches the not young description Osangar gives himself. Also, SH says they got the Gar's bodies from the Borderlands (Black hills are there)

Taim: "Dashiva is gaining his strength rapidly, but his head is in the clouds often As not. Even when it isn't, he is not always entirely there. Maybe he is just a day dreamer, and maybe he taint on Saidin is touching his brain already. . . ." Interesting that he gains strength rapidly

P. 85-86 Dashiva was in his middle years, a stocky fellow with a farmer's face. . . They (remaining ash/dedicated) did keep to themselves, though, and together except for Dashiva, who stood a few paces apart staring at nothing. Remembering Taim's caution about the fellow, Perrin hoped he was daydreaming.Perrin sensed something strange about him , RJ makes a point in the books of describing Dashiva as different than the others

P. 88 The man was not skinny, yet he moved in a hesitant, creeping way, hands folded at his waist, that made him seem so. "My Lord Dragon?" he said, tilting his head. "Can you make a gateway, Dashiva? Of course." Dashiva began dry-washing his hands, flicking at his lips with the tip of his tongue, and Perrin wondered whether the man was always this jittery, or just when speaking to the Dragon Reborn. "That is to say, the Mhael teaches Travelling as soon as a student shows himself strong enough." "The Mhael?" Rand said blinking. "The Lord Mazrim Taim's title my Lord Dragon. It means 'leader'. In the Old tongue." The fellow's smile managed to be nervous and patronizing at the same time. "I read a great deal on the farm. Every book the Peddlers brought by." Here Dashiva answers quickly to Rand's question, of course a Chosen can make a gateway, he realizes he answered to quickly, gets nervous and explains. He also uses the AOL word TRAVELLNG, Notice how he is jittery when talking to his old nemises LTT and Rand who killed him in EOTW. Then he slips by showing his Old Tongue knowledge, then again recovers by saying he reads.

Rand meets Cadsuane:

P. 328 Dashiva's face twitched as he glared from one Aes Sedai to another. . . Towards Cadsuane: Dashiva stared her up and down, and sneered . . .Dashiva's sneer turned to a grin, but a nasty one that made his former smirk lighthearted in comparison. "Do nothing." Rand snapped. Dashiva had the gall to glower at him before sullenly pressing a fist to chest in the salute the Ashamn used... Dashivas face twitches and he touches his face all the time. Osangar still is getting used to his new face. Remember when he first got it above, he cant resist touching his face. Also, Dashiva always sneers at AS, the chosen think of them as mere girls. Also, Dashiva had the gall to glower at Rand. Not what an Ashman would do to the DR.

P. 331 (After Cadsuane tells them men channelers hear voices) Startlingly, Dashiva gave a raucous laugh, shoulders shaking. As someone who is immune from the taint, Aginor finds thiseffect amusing.

Rand getting healed by Flinn:

p. 572=73 Dashiva made a guttural sound. "I assure you," Dashiva said, managing to sound oily and harsh at the same time, "Flinn knows what he is about. Already he can do things you Aes Sedai never dreamed of." How do you know this Dashiva? "Get on with it, Flinn," Dashiva muttered. "If he dies . . . . " Nose wrinkled as though at a bad smell, he seemed unable to look away from Rand. His lips moved as he talked to himself, and once he made a sound, half sob, half bitter laugh, without his face changing one line . Aginor doesn't want Rand to die, he may be held responsible. He fears what the DO would do this time, he would rather grovel before a worm than die again. Yet, he still laughs seeing Lews suffering like this.

P. 574 Dashiva nodded self importantly. "Yes he has a chance, now." You would have thought he had done the healing himself. Perhaps he did heal him. Aginor is a scientist. And if he sucks in healing like he claims, how does he know that Rand hasa chance now?

Rand and his boys prepare to go to Illian:

P. 640 After Cadsuane is rude to Rand Dashiva says: Dashiva barked a rough laugh. "If you want us to clear the women out . . . " The plain faced man began preparing flows not shields, but complex weaves of Spirit and Fire that Rand suspected would put anyone they were laid on in too much pain to think of channeling. Rand has pointed out that Dashiva was almost as strong as himself (Like Aginor is according to the Guide) and look at the fancy weaves he can form...)

P. 642 As the aes sedai leave: Dashiva laughed, a sharp wheeze, shaking his head he actually sounded amused. (Rand tells him to meet him in the anteroom) The plain faced Ashaman gave a start. He had been frowning at Adley. "As you command My Lord Dragon." Dashiva is amused, Osangar mentions above that he prided himself with his ability to have a sense of humor.

P. 643 All the Ashaman jumped when Rand walked into the anteroom. All except Dashiva . .. he then blinked several times before rising. Still getting used to that face, not used to jumping for anyone

P. 644 "I don't understand any of this (Plan to attack Illian) Dashiva sounded upset a muscle in his cheek was twitching. KEY (Dashiva goes on to tell Rand what he should do instead and what Sammael would do tactically.) Very interesting. Dashiva knows what sammael would do, he also seems botheres by the other fellow's weak plan, he cant allow Rand to lose or he would getsmoked. RAND: "Your all going to help me kill Sammael today." Only Dashiva looked startled. . Interesting turn of events in his mind, he would help kill another Chosen

P. 645 (After Rand is saying bye to Min, he sees Dashiva watching them and licking his lips while looking at Min. "For some reason the plain faced man raised his hands defensively. (Rand opens a Gateway,) Dashiva leaped back as it opened. Like at Shayol Ghul, Osangar reacts by raising his hands. He licks his lips getting used to his new face, and again he is called "plain faced" like RJ describes Osangar and Dashiva overand over again.

P. 648 When they arrive in the Courtyard in Illian, all the Ashamn and Rand are making fireballs, Dashiva is making Blue Lightening. RJ again makes Dashiva different. He makes lightening. Have we ever seen any of the Ashaman make this?
MISC:

Somewhere it says Dashiva cant use a sword at all. Aginor was a scientist, not a warrior. He held no field commands. RJ always says "plain faced" for Osangar and Dashiva. His twitching, hand rubbing, face touching, lip licking and muttering constantly remind us this is Osangar in his new body. The guide says Aginor's strength rivaled LTT and ISHY, weird that Dashiva has strength close to Rand.

CONCLUSION: Dashiva is Osangar is Aginor.