It's been five weeks since Oar left English Championship side Ipswich Town, citing homesickness as the reason.
Sydney FC as well as a number of other A-League clubs were believed to be in the hunt, but the Gold Coast-born Oar has opted for Roar, where he played 23 times, scoring two goals, between 2008-2010.
After leaving Brisbane, Oar headed to the Netherlands where he played 117 times for Eredivisie side FC Utrecht before moving to Ipswich where he made only nine appearances in half a season.
In a statement released by the Club, Football Operations Manager Craig Moore hailed Oar as a "quality player".
"His signing continues Head Coach John Aloisi's intentions to bring the best and brightest players to the club," Moore said.
Brisbane also confirmed that he would attend their next match against the Western Sydney Wanderers but would not play due to a lack of match fitness.