Chambers and Partners describes John O’Sullivan as a “strong presence” in the courtroom who is "brilliant at breaking down complex problems so they are easy to understand." He "easily straddles cultural divides and is on everybody's wavelength." John is listed in "Best Lawyers in America," “Super Lawyers,” “Florida Trend’s Legal Elite,” and numerous other guides identifying top lawyers. John has more than 20 years of experience litigating a wide range of complex commercial matters, including breach of contract and tortious interference claims; patent, trademark, and copyright litigation; trade secret and Lanham Act actions; unfair competition, price fixing, and monopoly claims; real estate, banking and construction litigation; securities and corporate governance disputes; and professional negligence and malpractice matters. He has also received numerous awards and recognition from legal services organizations for his efforts on behalf of pro bono clients.

During law school, John was a Trustee Scholar and a member of the George Washington University Law Review. He served as a law clerk to The Honorable William H. Timbers of the U.S. Court of Appeals for the Second Circuit and practiced for several years at a top New York City litigation firm before returning to Florida in 1998.