The 24-year-old has signed on for the remainder of this season plus two more years.
He will attend the Roar's Hyundai A-League Round 22 match against Western Sydney Wanderers at Suncorp Stadium on Friday but will not feature in the clash.
The Gold Coast-born Oar has played 34 times for Australia and scored four goals.
He had two seasons for the Roar from 2008-10, playing 23 times for two goals.
Oar then headed overseas where he played 105 games (14 goals) for Dutch club FC Utrecht.
Since deciding to return to Australia last month, Oar has been hotly chased by a number of Hyundai A-League clubs but committed to the Roar today.
"Tommy is a quality player with a proven record in top-flight football," said the Roar's Football Operations Manager Craig Moore.
"His signing continues Head Coach John Aloisi's intentions to bring the best and brightest players to the club."
This article was originally published at: http://www.brisbaneroar.com.au/article/oar-signs-with-roar/123v09ss863h01dfxtksghx1t1.