How does O'Brien discredit Osvaldo's claim that he is afraid of Bobo? How else does she discredit his testimony in general?

While on the witness stand, Osvaldo claims that he participated as a lookout boy in the robbery because he was threatened by Bobo. Osvaldo also explains that he is testifying because he is getting a deal from the government. During Osvaldo’s cross-examination, O’Brien brings up the fact that Osvaldo is a member of a violent street gang called the Diablos. This point casts doubt on Osvaldo’s innocence and his alleged fear of Bobo.