There’s an old legal saying: “When the facts are on your side, argue the facts, if the facts are not on your side, argue the law, if neither the facts nor the law are on your side, pound on the table”.
A lot of legal commentors remarked on the contrasting closing arguments in that they were a reversal from the norm; the prosecution went for the emotional rhetoric and hypothetical scenarios and the defense was calmly arguing facts. But that’s just because of the position they were in. O’Mara could argue facts, and the prosecution had to pound on the table. Most of the time, the defendant is guilty, so it’s generally the other way around.
Put O’Mara in the more customary position of defending a guilty client, and it would be a better test of his abilities.