eeeeehhh... Oro told Kabuto that Itachi was too strong for him after the battle with konoha.... what other proof is needed? he left the organization 7 years in the past after that battle with Itachi.. hence Itachi was stronger back when he was 11 and MUCH stronger than him 10 years later as we saw on Itachi vs Sasuke.
Besides... all this are speculations of what Oro could have done but the fact is what happened. Oro got paralyzed with genjutsu he was gonna try to do something and Itachi literaly CUT that chance of counter to zero.
An seriously... if itachi wanted he could had killed Oro that day... but knowing his nature he probably let him go after he was no longer a danger for him.