O’Melveny & Myers partner Daniel Petrocelli is used to litigating in the limelight, from suing O.J. Simpson to defending Jeffrey Skilling.

But as one of the biggest trials of his life—fending off the Justice Department’s challenge to AT&T’s $85 billion merger with Time Warner— kicks off Monday, there’s a niggling question: What does the Los Angeles-based lawyer really know about antitrust law?