[2] On 7 September 2021, Guy successfully challenged O'Brien for the party leadership, and returned to the role of opposition leader.

With co-author Jamie Richardson, O'Brien won the Law Institute of Victoria's Rogers Legal Writing Award in 2006.

[5] While at the Bar he also lectured part-time in trade practices at the Leo Cussen Institute of Continuing Legal Education and performed pro bono work.

[6] He stood for the Liberal Party in the blue-ribbon seat of Malvern at the 2006 state election, winning with over 60 per cent of second-preference votes.

[citation needed] In April 2009, he called for the Brumby Government to change company share laws so that apartment owners could take consumer cases to the Victorian Civil and Administrative Tribunal, instead of having to go the Supreme Court.

[10] He told a local newspaper that he was "delighted with the changes" and was looking forward to "developing a better alternative to Labor's poor performance [in energy security and improved infrastructure]".

[13] After the move of Liberal MP Geoff Shaw to the crossbenches and the resignation of Premier Ted Baillieu in March 2013, O'Brien became Treasurer of Victoria in the ministry of the Napthine minority government.