I am a bigger fan or Kakashi, but Oro would win, hands down. He was arguably the strongest Sannin and he was considered Kage level by the 3rd Hokage. Oro is MUCH faster, and any use of Kakashi's MS would deplete his chakra reservoirs too much. His MS is just a chakra killer with few additional perks. And as for his dog summons, I think the giant snake summons would completely obliterate them. There's no way Kakashi would be able to fight against 4 tailed Naruto. J-Man was barely able to do it, and Kakashi is NOT on his level. Kakashi would have been a horrible choice for Hokage to me. When compared to the other Kage, his stamina is too low and he always get's hurt in his fights, like the fight with Zabuza, his fight with Itachi, his fight with Kakuza. You get my drift?
Kakashi is apparently more likeable. But Oro would win EASILY.