Buck was the first black coach in MLB history, serving as one of the many rotating coaches during the College of Coaches era. Before coming to the Cubs he was a player and later manager in the Negro Leagues. He was Ernie Banks' manager when Ernie was with the Kansas City Monarchs. My guess is that was a factor when the Cubs hired him, first as a scout, and later as coach.
His appearance in Ken Burns' Baseball documentary brought him to national prominence. For the rest of his life O'Neil was the unofficial spokesman for the Negro Leagues.
He died at the age of 94 in 2006.