I'm not sure the ease with which he was killed both times is a factor. However, at this point I don't see any strategic reason for the DO to resurrect him again, unless some of his scientist skills are needed. It's probably too late in the game to place him as an undercover agent somewhere, as he was as Dashiva. And the DO doesn't really need him as a weapon during TG, now that he has a bunch of Dark Asha'man at his disposal. And Aginor was never a key general or combatant during the AoL, from what we know, so his usefullness as such is questionable, anyway.

So I don't see him returning either, even though I think the DO was able to capture his soul. We have no reason to believe otherwise - Ishamael was killed with callandor, and the DO was able to capture his soul. So he ought to have been able to capture Osan'gar's soul too.