Trimble has been in impressive form for Ulster this season and O'Sullivan has decided to give the powerful 21-year-old his chance alongside Leinster's Gordon D'Arcy.
O'Sullivan said: "Andrew is ready for international rugby. He's an outstanding player and his form has been exceptional.
"I thought New Zealand was not the best game to give him his first Test start but I had to use him this autumn and this is the best time.
"Defensively he's strong. He's very gutsy, quick and a good ball carrier who likes to take players on and run at them."
Anthony Bowe pays the price for Trimble's elevation to the Test team with the Munster winger dropping out of the squad after a nervy display against New Zealand.
British & Irish Lions back Shane Horgan is moved out to the right wing while Ulster's Tommy Bowe is switched to the left wing.
Llanelli flanker Simon Easterby will lead the team for the second time despite sustaining a perforated lip against the All Blacks.
Peter Stringer has also won his fitness battle. The Munster scrum-half left Lansdowne Road on crutches on Saturday but his bruised heel has responded well to treatment.
O'Sullivan added: "Simon is fine. He's on track provided he doesn't get another bang on the area. Peter had a badly bruised heel but we don't see any problem with him."
Australia were beaten 26-16 by England last Saturday and are desperate to avoid their eighth straight Test defeat - the worst sequence of results in their history.
Ireland team to face Australia at Lansdowne Road on Saturday (1430):
G Murphy (Leicester); S Horgan (Leinster), A Trimble (Ulster), G D'Arcy (Leinster), T Bowe (Ulster); R O'Gara (Munster), P Stringer (Munster); M Horan (Munster), S Byrne (Saracens), J Hayes (Munster), D O'Callaghan (Munster), M O'Kelly (Leinster), S Easterby (Llanelli, captain), J O'Connor (Wasps), D Leamy (Munster).
Replacements: R Best (Ulster), S Best (Ulster), M McCullough (Ulster), N Best (Ulster), K Campbell (Ulster), D Humphreys (Ulster), G Dempsey (Leinster).