Ulster star Andrew Trimble will make his international debut when Ireland face Australia in Dublin on Saturday.
The 21-year-old theology student is the only change to the side defeated 45-7 by New Zealand last week.
Trimble will partner Gordon D'Arcy in midfield with Shane Horgan moving to the wing in place of Anthony Horgan.
"I was always thinking of Andrew for this game," said O'Sullivan. "He has played well enough this season to warrant a place in the starting side."
A product of Coleraine Academical Institution, the alma mater of former international Jonny Bell, Trimble represented both Ulster and Ireland at schools and U21 level.
Trimble, who studies at the University of Ulster in Jordanstown, charged on to the scene at the end of last season when he starred in Ireland's U21 World Cup side.
He played for the Barbarians against the Combined Services last November and scored two tries in a 38-36 defeat.
Born: 20 October 1984
Height: 6ft 2in
Weight: 14st 6oz
Teams: Ballymena, Ulster, Irish U21, Barbarians
Trimble made his full Ulster debut this season, making an immediate impression with his strong, intelligent running. On Saturday he must fill the shoes of his and the entire country's hero, the injured Brian O'Driscoll.
But perhaps the biggest surprise about Trimble's inclusion is that he is the only change to a team that was so comprehensively beaten by the All Blacks.
The pack remains intact, despite coming under extreme pressure from the New Zealand eight, with skipper Simon Easterby having recovered from a mouth injury.
Scrum-half Peter Stringer also came through a fitness test to continue a long-established link-up with his Munster colleague Ronan O'Gara, who resisted pressure from Ulster's David Humphreys to keep his place.
O'Sullivan appears to have kept faith with the struggling O'Gara largely because the 34-year-old Humphreys is unlikely to be an option for the 2007 World Cup as his contract ends next June.
O'Sullivan said that he resisted the move to make more changes to his side.
"I could have made another couple of changes but I thought I'd give the team another crack," said O'Sullivan.
"The boys who came on against New Zealand did well, so there is room to manoeuvre. But a raft of changes wouldn't have done anything for the team.
"In our situation it was good to take a deep breath and look at our performance against New Zealand. We weren't terrible, they were just outstanding."
Ireland: G Murphy; S Horgan, A Trimble, G D'Arcy, T Bowe; R O'Gara, P Stringer; M Horan, S Byrne, J Hayes, D O'Callaghan, M O'Kelly, S Easterby (capt), J O'Connor, D Leamy.
Replacements: R Best, S Best, M McCullough, N Best, K Campbell, D Humphreys, G Dempsey.