Brigadier Tim O'Brien was raised and educated in Brisbane, Queensland. He enlisted as a Reserve soldier in the Queensland University Regiment (QUR) in July 1983 and has serves continually as an Active Reservist since that time.
Upon graduating from QUR in 1988, Brigadier Tim O'Brien was allocated to the Royal Australian Artillery and, from 1988 to Dec 2000, held several regimental appointments within 5th/11th Field Regiment, and its successor Regiments, culminating in his posting as Battery Commander of the 41st Field Battery, 1st Field Regiment, from January 1988 to December 2000.
Brigadier Tim O'Brien has held three Training Establishments postings, the first of which was to the Regional Training Centre (Sth QLD) in 2001 and 2002 as a tactics instructor and then as the Senior Instructor of the Intermediate Staff Course; the second was as the Chief Instructor at LWC (Sth QLD) in 2008, and his third was as the Commanding Officer of QUR from January 2009 to December 2011.
In 2012, Brigadier Tim O'Brien was appointed to the position of Colonel-Plans within Headquarters 11th Brigade, and from January 2014 to December 2015 held the appointment of Deputy Commander of 11th Brigade. During 2016, he held the posting of Colonel Reserves at Headquarters Forces Command.
Brigadier Tim O'Brien was promoted to his current rank in November 2016, and held the appointment of Commander 11th Brigade, from January 2017 to December 2018. His professional qualifications include a Bachelor of Commerce, a Graduate Diploma in Corporate Finance and a Graduate Diploma in Applied Corporate Governance. He is a Chartered Accountant, a Chartered Secretary and a Fellow of the Financial Services Institute of Australasia.