Mr O'Brien lost the Victorian Liberal leadership after declining to contest a spill last September and did not feature in Mr Guy's initial shadow cabinet.
But the Malvern MP will return to the opposition frontbench in the post of shadow attorney-general as part of a pre-election shake-up announced on Friday.
Nine months out from the state election, Mr Guy flatly rejected his predecessor's return could create simmering tension in the Liberals and Nationals partyroom.
"Michael wants us to win the election," he told reporters.
"Don't ever doubt his or my primary goal for being in this building is to get a coalition government ... in office at the next election."
In a statement, Mr O'Brien said he had accepted the role in a bid to hold the Andrews government to account for its management of the legal system.
"Victoria will never recover and rebuild whilst we suffer the worst criminal court backlogs in the nation," he said.
The opposition leader said his new shadow attorney-general was well-suited to the role, having worked as a barrister and legal counsel to then-federal treasurer Peter Costello before entering Victorian parliament in 2006.
"Michael knows that portfolio from back to front," Mr Guy said.
After returning for a second stint as opposition leader last year, Mr Guy's close factional ally Tim Smith was picked as shadow attorney-general despite not having a legal background.
The Kew MP resigned from the role in October and moved to the backbench after crashing his Jaguar into the side of a Hawthorn home and blowing more than two times the legal blood-alcohol limit.
Upper house Liberal MP Matthew Bach, who has also never been a lawyer, was selected as his replacement but has vacated the post for Mr O'Brien.
Mr Bach remains a shadow cabinet member and will take over the transport infrastructure portfolio from Ripon MP Louise Staley, who will become spokeswoman for government scrutiny.
Forest Hill MP Neil Angus also kept his shadow assistant minister positions, despite currently being unable to attend state parliament due to being unvaccinated.
Mr Guy confirmed he doesn't intend to change his shadow cabinet again before the state election on November 26.
7°C