Gordon D'Arcy, who made his 75th Heineken Cup appearance in the quarter-final win over Leicester Tigers, and Jamie Heaslip are both expected to be available for selection.
Isa Nacewa should also be back in contention after he was left out of the squad for the first time this season for the trip to Viadana.
Back rower O'Brien lies in joint third place in the Heineken Cup's top try scorers list this season with four tries.
Meanwhile, Brian O'Driscoll (29 tries) is joint-second top try scorer in the tournament's history, three tries behind Toulouse winger Vincent Clerc.
Head coach Joe Schmidt is set to reveal the team and replacements to face Toulouse on Friday afternoon.