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Attorney Dennis O'Brien

Dennis O'Brien

Experienced Criminal Defense Lawyer in Long Island

Dennis O’Brien is the founder of O’Brien Law Group, LLC. Prior to founding O’Brien Law Group, LLC, Mr. O’Brien was a partner in a prestigious Garden City, NY Law Firm for twelve years. Mr. O’Brien’s focus during that time was criminal defense. He also was an Assistant District Attorney with the Nassau County District Attorney’s Office. Serving as a prosecutor from 2002 until 2006, Mr. O’Brien handled all kinds of criminal matters including the most serious violent felonies.

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Mr. O’Brien has significant experience in the realm of criminal practice. For the past sixteen years, he has handled hundreds upon hundreds of criminal cases to disposition. He further has substantial litigation experience. Mr. O’Brien has vast experience in advising clients testifying before the grand jury, litigating pre-trial suppression hearings, and trying criminal cases to verdict. In private practice, Mr. O’Brien also has amassed vast experience with Department of Motor Vehicle (DMV) hearings. Mr. O’Brien has handled numerous refusal and fatality hearings and the corresponding departmental appeals procedures.


In 2016 Mr. O’Brien was recognized by the members of the Fraternal Order of Court Officers (FOCO), which is an organization of New York State Court Officers and Court Clerks, with the Amicus Fraternitas Award, which is awarded to a member of the legal community based on his professionalism, dedication and integrity.

Mr. O’Brien was elected and served as a member of the Board of Directors of the Criminal Courts Bar Association of Nassau County from 2008-2016. During his time on the Board of Directors, Mr. O’Brien chaired the Legal Action Committee. In this capacity, he authored several Amicus Curiae briefs on behalf of the Association. In 2015, he was elected President of the organization and dutifully served in this office, frequently participating in meetings with senior representatives of the Office of the District Attorney and the Supervising and Administrative Judges of the Courts of Nassau County.

Mr. O’Brien is an Adjunct Assistant Professor at St. John’s University and has been teaching courses on the law for the past eighteen years. He also continues to present his academic writing at colloquia and conferences throughout the country.

Mr. O’Brien received his J.D. from Brooklyn Law School, M.A. from St. John’s University, and B.A. from St. John’s University. He is also a graduate of Chaminade High School. Mr. O’Brien is a member of the Nassau, Suffolk and New York State Bar Associations, the Criminal Courts Bar Association of Nassau County, and the National Association of Criminal Defense Lawyers, and is admitted to practice law in New York State and the Eastern and Southern Districts of New York.


  • Juris Doctorate (J.D.) - Brooklyn Law School 2002
  • Master of Arts (M.A.) - St. John's University 1999
  • Bachelor of Arts (B.A.) - St. John's University 1998

Bar Admissions

  • New York State
  • Eastern District of New York
  • Southern District of New York


  • National Association of Criminal Defense Lawyers
  • New York State Bar Association
  • Suffolk County Bar Association
  • Criminal Courts Bar Association of Nassau County

Awards & Accolades

  • 2018 – Appointed Vice Chair, Criminal Law and Procedure Committee, Nassau County Bar Association
  • 2017 – Appointed Vice Chair, Criminal Law and Procedure Committee, Nassau County Bar Association
  • 2016 –Amicus Fraternitas Award – FOCO
  • 2015 – Elected President – Criminal Courts Bar Association of Nassau County
  • 2008-2016 – Elected Member of the Board of Directors of the Criminal Courts Bar Association of Nassau County
  • 2000 to Present – Adjunct Assistant Professor, St. John’s University

    • Courses Taught: Constitutional Law, Judicial Process and Behavior Comparative Law, International Law

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