The fact is, Itachi is the strongest nin shown thus far in the series or manga. Oro in his prime is NOT as strong as him. Itachi while he was playing around was too much for a couple jounin. Not once has Itachi been serious except when he decides the current situation is too much of a hassle. His exploding kage bunshin caught everyone, save Kakashi, off guard and it hurt kakashi a fair bit. We have already seen Oro fight seriously and it is not too impressive. The most he has done was rely on his jutsu. Itachi, on the other hand has one thing above Oro which is definite, his intellegence. He easily tricked Kakashi with the shuriken in his hand, and then snuck up on Kakashi's Mizu Bunshin and stabbed it. He is very very sneaky and smart. Even if the unprobable is true (that he is weaker than Oro), he could most likely still beat him due to his intellegence. Who knows what other tricks he has up his sleeve.. or what his potential is when he is serious?
Sharingan is a good bloodline, granted. BUT it takes a hell of alot more to be an excellent ninja. Notice how Oro was not afraid of Kakashi at all even though Kakashi has a sharingan (although he cannot do that special genjutsu thing).
EDIT: However, I think he is stronger than Sarutobi, yet Saindaime would be able to beat him
