Oliver Queen's and Lex Luthor's reason for animosity towards each other is revealed in this interesting episode of "Smallville."
One of Oliver's high school buddies mysteriously gets murdered at their high school reunion, the same boarding school that Lex also attended. Oliver blames Lex for the friend's death, but there's much more behind the real killer. This revelation manifests itself after several flashes to the past involving the school rivalry of the bully Oliver, his two chums, against Lex and his friend Duncan, who Lex betrays in an attempt to fit in.
This episode is interesting because it creates a new backstory to one of the show's key characters and a new key character for this season.