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Thomas P. O'Brien

Partner, Litigation Department
T 1(213) 683-6146
F 1(213) 996-3146

Thomas P. O’Brien is a Litigation partner with Paul Hastings in Los Angeles. Prior to joining Paul Hastings, Mr. O’Brien served as the United States Attorney for the Central District of California, after being appointed by President George W. Bush and confirmed by the United States Senate. He routinely advises and represents corporations in high-profile federal and state investigations, including federal and state False Claims Act (also known as qui tam matters), Foreign Corrupt Practices Act (“FCPA”), and other corporate compliance matters, and oversees complex civil and criminal litigation matters.

Mr. O’Brien has vast experience as a trial attorney and has tried over 95 cases, both criminal and civil, in federal and state court. He has represented national and multi-national companies in civil actions with billions of dollars in dispute, and has successfully represented individual corporate leaders and foreign heads of state in civil, criminal, and regulatory proceedings. He has experience navigating dual civil-criminal proceedings and handling complex, first-of-its-kind matters in multiple jurisdictions.

In addition, Mr. O’Brien represents corporations and individuals at all phases of government investigations and prosecutions, guiding his clients from their first contact with the DOJ, SEC, state or regulatory authorities through trial. He regularly counsels clients as they conduct internal investigations into potential violations of law or other wrongdoing. He also investigates and advises clients on embezzlement, accounting fraud, and other complex white collar matters.

During his tenure as U.S. Attorney, Mr. O’Brien was responsible for federal criminal and tax investigations and prosecutions and all civil matters involving the United States, including civil fraud and False Claims Act matters, in the largest federal district in the country. Under his leadership, the office filed the most felony cases in the history of the district, and increased complex case filings by 50 percent. The office also filed the largest RICO case ever against a motorcycle gang and, for the first time in the nation, seized that gang’s trademark.

Before he was confirmed as U.S. Attorney, Mr. O’Brien served for more than two years as the chief of the Criminal Division. In that capacity, he oversaw several high-profile espionage cases, helped bring the first treason case in the nation since World War II, supervised several high-profile public corruption and civil rights cases, charged several cyber crime statutes which had not been charged in the nation, and executed some of the largest fraud cases in the United States. He was the only criminal chief in many years who tried cases during his tenure.

Before serving as Criminal Chief, he was an Assistant United States Attorney for four years before rising to Chief of the Civil Rights Section, where he personally investigated and prosecuted numerous cases, including federal hate crimes, human trafficking violations, and police misconduct cases.

Recent Representations

  • Mr. O’Brien represents a major national services corporation in ongoing negotiations with the Department of Justice concerning allegations that the company violated the False Claims Act. 
  • Mr. O’Brien represents a foreign conglomerate in a high-profile criminal matter involving allegations of international trade secret theft pending in the Eastern District of Virginia.
  • Mr. O’Brien represents a major national health care network in an ongoing investigation by the Department of Justice into potential violations of the False Claims Act.  He also leads an internal investigation into those allegations at the company and its affiliates.  
  • Mr. O’Brien currently leads the defense of a university professor in a high-profile felony prosecution involving alleged labor code violations. The case has garnered national media attention in what has been described as the first case of its kind. 
  • Mr. O’Brien led a team of attorneys in conducting an internal investigation at the request of a Southern California health center. That investigation, since closed, arose out of allegations that employees of that health center accepted improper benefits in exchange for referrals to certain patient facilities in violation of federal and state anti-kickback statutes. 
  • Mr. O’Brien represents a hospital in connection with an investigation by the Department of Justice and Office of the Inspector General into potential violations of the False Claims Act and federal Anti-Kickback Statute. 
  • Mr. O’Brien represents a physician with a successful medical practice who is being investigated by the Department of Justice for claims of fraudulent billing. When Mr. O’Brien was brought onto the case, the investigation had been ongoing for several years, and the government had issued a final plea offer and expressed their intention to indict the client. Within weeks, Mr. O’Brien and his team conducted an investigation and presented new evidence to the Department of Justice. The Department of Justice decided to allow the statute of limitations to expire on the charges that they, only weeks earlier, intended to file.
  • Mr. O’Brien represents a former CEO against allegations that the client engaged in insider trading in connection with a secondary offering. 
  • Mr. O’Brien represents a medical device manufacturer against allegations that it violated the False Claims Act.
  • Mr. O’Brien represented the police chief of a local city who faced civil charges filed by the state Attorney General as part of allegations against an entire city council and several administrators for defrauding taxpayers and wasting taxpayer monies. In a case described by numerous media outlets as one of the biggest corruption cases in the nation, and despite the extreme public outcry attacking his client, Mr. O’Brien and his team convinced the court to dismiss the complaint with prejudice.
  • Mr. O’Brien represents a local airport commission in an investigation by the Federal Bureau of Investigation into allegations of corruption. Within months of being retained in this case, Mr. O’Brien managed to focus the government on the actions of other individuals and received a letter from the Department of Justice clearing his client.

Accolades and Recognitions

  • Received awards from many different organizations, including the Los Angeles City Council; Los Angeles Police Department; Federal Bureau of Investigation; Drug Enforcement Administration; United States Secret Service; Internal Revenue Service; United States Marshal’s Office; Santa Barbara District Attorney’s Office; United States Postal Service; United States Postal Inspectors and Office of Inspector General; Bureau of Alcohol, Tobacco, Firearms and Explosives; and the Los Angeles City Attorney
  • Received the 2007 Anti-Defamation League Pacific Southwest Region’s Helene and Joseph Sherwood Prize for combating hate
  • In October 2007, received the Attorney General’s Award for Exceptional Service—the highest award given by the Attorney General—for his role in investigating and prosecuting street gang members who committed hate-crime murders of African-Americans in Northeast Los Angeles
  • Received the 2008 Immigration and Customs Enforcement Assistant Secretary’s Award for Excellence in Law Enforcement

Professional and Community Involvement

  • Member of the State Bar of California
  • Serves on the Board of the Chandler School in Pasadena, California
  • Sat on the President’s Corporate Fraud Task Force and was Chair of the Attorney General’s Advisory Committee’s Cyber/Intellectual Property Subcommittee
  • Member of the Legal Advisory Committee for the Anti-Defamation League
  • Served on the Board of the Legal Aid Foundation of Los Angeles (LAFLA)
  • Vice-Chair, International Anti-Corruption Committee, Steering Committee, Asia Regional Forum


  • University of San Diego School of Law, J.D., 1993 (cum laude, Associate Editor of the San Diego Law Review)
  • United States Naval Academy, B.S., 1981