I saw a tiny speck of a shadow in Oblivion of a distant tower and 1shotted a guard of said tower with a headshot. It was using a "skill/arrow quality = arrow speed/trajectory" mod and an "extra ranged damage sneak attack damage", but nothing that would make such a shot a sure thing.
I also shot the mage in the thieves quest by just hearing his voice.. I heard him talking and was like "I bet you he's just.. THERE" from a lower level, fired my arrow, killed him in one shot.