If you want to target someone in a mob scene you would take the tallest guy around, put a distinctive head gear on him and let Oswald go in front.
Like, say, this picture, the man to his right is taller than the others. His tall cowboy hat makes him even more noticeable and he is letting Oswald be completely vulnerable from the front.
Ruby comes out of the crowd at exactly the right moment. Back in the crowd he cannot see where LHO is until he spots the cowboy hat and moves toward it and Oswald like radar.
and there were no cops in front of him allowing easy kill shots.