He was commissioned an Ensign through the Navy’s Limited Duty Officer program on March 1, 2004.
In April 2004, LCDR O’Brien reported to USS BELLEAU WOOD (LHA-3) as Ship’s Bosn and earned his Surface Warfare Officer designation. During the ship’s 2004 Western Pacific, he was assigned as ship’s First Lieutenant, and subsequently led Deck Department through the ship’s decommissioning in October 2006.
In November 2006, he became Navy Region Mid-Atlantic Deputy Director of Bachelor Housing, where he managed the Bachelor Housing Public Private Venture (PPV) program transition and the merger of Navy Regions Northeast and Mid-Atlantic bachelor housing programs.
From March to November 2007 he completed an Individual Augmentee assignment with CJTF HOA onboard Camp Lemonier, Djibouti, Africa.
In November 2007 he was assigned as Officer-In-Charge, Correctional Custody Unit and Executive Assistant to the Commanding Officer of Naval Brig Norfolk.
In December 2007 he reported to Naval Beach Group TWO at Naval Amphibious Little Creek as Officer-In-Charge, Naval Support Element Detachment Charlie, and deployed with the IWO JIMA Expeditionary Strike Group.
In December 2009, LCDR O’Brien reported to Commander, Navy Installations Command Headquarters, as Deputy Director, Port Operations, and in July 2010, he was assigned as Officer-In-Charge, US Navy Oil Spill Response In-Shore Recovery Group West.
From December 2009 to September 2011, he served as Officer-In-Charge, Assistant Operations Officer and Operations Officer at Assault Craft Unit FOUR, and has served as Chief Staff Officer and Operations Officer for Naval Beach Group TWO since May of 2015.
He is entitled to wear the Meritorious Service Medal, Navy Commendation Medal (6 awards), the Navy Achievement Medal (3 awards), the Good Conduct Medal (three awards) and various service, unit and campaign awards.