Dr. Faraj hits Robson's mouth with a phone and tells officers that Robson attacked him and sneered and walked out.
Why did he have such behavior?
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According to the Oz wiki, it's pretty clear why Dr. Faraj hates Robson. Dr. Faraj is of Pakistani origin and Robson is a white supremacist, who made a ton of racist remarks at Dr. Faraj.
Dr. Faraj wanted revenge on Robson, so when Robson received gums from a dead person, Dr. Faraj is trying to scare Robson by claiming that the gums could have came from anyone including a Jew, Latino, Gay, or even a Black man.
He then pays Poet to spread the word about the gums that Robson has received and at lunch that day, he and Ryan O'Reily tell everyone about Robson's new "ghetto gums." Angry, Robson confronts Faraj, who then hits him in the mouth with a telephone and tells him the truth about his gums--he made 100% sure they did NOT come from a Caucasian, as revenge for Robson's racist behavior.