Not trying to defend WC, but Oswald could easily have walked away unnoticed. It was total mayhem immediately after the shots were fired. I was on the corner of Main and Houston when it happened. People were totally confused and were going every which way. A number of people, including myself, immediately left the area. (I walked back down Houston Street to the Dallas Morning News building where I had earlier gone to apply for a part-time job.)
Of course, no one could know what was going through Oswald's mind, but he probably wasn't thinking clearly. That may account for him getting on the bus. If he was trying to get to his rooming house in Oak Cliff, he had no choice but to get on the bus headed back toward Dealey Plaza. The "Marsalis" bus would have turned left (south) at Houston Street then across the Houston Street viaduct to Oak Cliff. The traffic jam on Elm Street was all the prevented his getaway on the bus.