There is a report that soon after the assassination as he was leaving, Lee Oswald interacted with Jack Ruby outside the TSBD. There were some children who saw it, and one of them reported it to her mother, who spoke to police. For obvious reasons, it was never followed up.
I rate the chances of it having happened as very good. I don't think there is any doubt that Jack Ruby was in Dealey Plaza during the shooting. The whole idea that he was at the office of the Dallas Morning News is just plain ridiculous. He was ID'd by multiple persons in the Plaza before the motorcade, including delivering weapons. Julia Ann Mercer is one such person who saw him.
And don't forget that there is also Fedora Man in the Altgens photo, and I rate the chances of him being Jack Ruby as extremely good. I also think that obscuring him was the reason that the Altgens Woman and Boy were added to the photo. And they were definitely added because she could not possibly be holding him to give him the elevation that he undeniably has.
And there is absolutely no question that Ruby and Oswald knew each other. Multiple witnesses saw them together in multiple locations at multiple times. Therefore, if Ruby was lingering outside after the assassination (and Victoria Adams is another person who saw him; she said he was "barking orders and acting like a cop") and if Oswald came out to leave and he saw Ruby, don't you think they would have exchanged a few words, especially after what happened?
But, my question to those who think that LHO was the figure Roger Craig saw scurrying to the Nash Rambler, getting in, and darting off, what do you make of it? According to Craig, Oswald came running out and went right to that waiting car. It leaves no time for him to converse with Jack Ruby.