How did Kathy O'Brien defend Steve Harmon?

If you mean the manner in which Kathy O'Brien defends Steve Harmon, the answer is that she fights diligently to win an acquittal for her client. If, however, you are referring to O'Brien's tactics, she basically presents Steve as a naive teenager who was hoodwinked into helping Bobo and King commit a crime.
In court, O'Brien takes pains to portray Steve as a young man who was framed. Her modus operandi is to normalize Steve in the jury's eyes. To that end, she calls Steve's film teacher, Mr. Sawicki, to the stand. During his testimony, Mr. Sawicki maintains that Steve is an "outstanding young man" and that the latter is also "talented, bright, and compassionate." O'Brien calls on Mr. Sawicki to testify in order to reinforce Steve's image as an innocent victim.
Before the last stages of the trial, O'Brien warns Steve against trying to "act tough" like King or Bobo. She tells him not to hide his obvious anxiety about the trial. O'Brien maintains that Steve must reveal his "real" self to the jury; this is the only way he can win the court's sympathy.
During the closing argument, O'Brien reminds the jury that the state has not proven Steve's guilt in the matter of Nesbitt's death. Furthermore, the state has not been able to prove beyond a shadow of a doubt Steve's connection to Bobo and King. O'Brien also reminds the court that Bobo's testimony about the lookout signal proves nothing. After all, Bobo testifies that Steve left the drugstore without signaling that anything was wrong. During her closing argument, O'Brien takes further pains to distinguish Steve from King (and Bobo) who both feasted on fried chicken after Nesbitt's murder.
To finish her argument, O'Brien again highlights Steve's character. She details how he has done everything in his power to answer every question honestly and openly.
O'Brien tactically portrays Steve as a naive teenager who was tricked into taking the fall for Bobo and King's actions.