Thomas Peter O'Brien (born June 19, 1959) is an American white collar criminal defense lawyer at the Los Angeles law office of Ellis George Cipollone O’Brien Annaguey LLP.

[9] He is currently a litigation partner, where he focuses his practice on white collar defense and representing clients in federal investigations, at the Los Angeles law office of Ellis George Cipollone O’Brien Annaguey LLP.

[10] In 2009, O'Brien launched an inquiry of Cardinal Roger Mahony, the leader of the Roman Catholic Church's Los Angeles Archdiocese, which caused the Federal Grand Jury to take action.

Shortly thereafter, an investigation by the U.S. Justice Department Inspector General uncovered that O'Brien stayed at pricey hotels while traveling and then sought government reimbursement by telling his secretary to lie and state falsely that more affordable rooms were not available.

[14] However, as of November 2010, O'Brien made right with the case and has stated that "he didn't do anything wrong," but told the L.A. Times that he takes "full responsibility" for excessive spending caused by any errors.