Pete, I'm not saying Fredo would have been included in any business meetings between Vito and/or Roth. I'm saying that he might have seen them around Vito. In a deleted scene from II, we see Clemenza introducing the young Roth to Vito, who hired him as a truck mechanic. Surely all the Corleone children would have had opportunities to see/meet him. That's why, when Fredo said, "I don't know those guys" [emphasis added], Michael should have smelled a rat.
I don't know if it's entirely implausible that Ola and Fredo did not know one another. As Pete says, Fredo was hardly central to the Family business.
And their remarks about one another - Ola's "we never met" and the Fredo comment cited above - seem consistent with people who travel in the same circles but never had direct interactions.
Of course, they were lying and Michael was right to be suspicious, but I don't think it was so blatantly untrue that Michael could or should have known without a doubt that Fredo was the traitor.
