Why is Ms. O'Brien's cross examination of Osvaldo so strong in Monster?The book is Monster By Walter Dean Myers. I really need help.

Ms. O'Brien is the defense attorney, and her job is to make the jury see her client, Steve Harmon, in the best possible light.  As a witness for the prosecution, Osvaldo, who is placing the blame for the crime on Steve and two others, has been presented as an innocent young boy who was involved in the robbery out of fear of retaliation.  As such, he has come across as a sympathetic figure to the jury, and his testimony against Steve has been damaging.  In her cross examination, Ms. O'Brien is seeking to present a different side of Osvaldo to the jury, so as to discredit his testimony.  For this reason, she is very hard on him, challenging his statements in hopes of exposing the boy for the type of character he really is.

Although Osvaldo is young, he is a street-wise gang member.  Ms. Petrocelli has coached him to speak "softy, timidly" when he testifies, so that the jury will find him likeable and feel sorry for him.  Osvaldo claims that he was involved in the robbery only because he was threatened by Bobo, the mastermind behind the crime.  According to Osvaldo, he was afraid that Bobo would "cut (him) up", and "get (his) moms, too", if he didn't cooperate. 

Under cross examination by Ms. O'Brien, Osvaldo reveals that he was apprehended when he had a fight with his girlfriend because he had gotten another girl pregnant, and his girlfriend had called the police.  He also first denies then admits under questioning that he is a member of a gang, and says that in order to get into this gang, he had to "fight a guy who's already in the club to show (he's) got the heart", as well as cut a stranger in the face with a knife to "leave (his) mark" so that it shows.  Through Ms. O'Brien's sometimes harsh but skillful examination, Osvaldo is revealed to be the hardened gang member that he is despite his young age.  He is no longer the sympathetic character in the eyes of the jury that the prosecution has presented him to be, and his account is now questionable, especially since it has been further revealed that Osvaldo is being offered immunity for his testimony.  Through her strong cross examination, Ms. O'Brien has succeeded in discrediting what Osvaldo has to say, thus making things look better for her client.