O'Brien's last game was against France on 20 August

Eoin Reddan, Sean O'Brien, Rob Kearney and Cian Healy return for Saturday's key Pool C clash against Australia.
European player of the year O'Brien is named at open-side after completing his rehabilitation from a knee injury which has kept him sidelined since 20 August.
Kearney has not played for a month due to a groin strain but is fit again.
He replaces Geordan Murphy at full-back while Healy is fit again after sustaining an eye injury against England two and a half weeks ago.
Apart from the absence of David Wallace, who was ruled out of the World Cup after suffering a knee injury in the warm-up defeat by England, Kidney has named his strongest side to face the Wallabies.
"The players coming back will bring freshness to the team," Kidney said.
"If you play the same team week in, week out, it becomes difficult for everybody.
"I didn't think Sean, Rob or Healy were as fit as they needed to be to play against the USA last week so they got another week under their belts.
"They're raring to go and that brings its own impetus into the team. That's what you need."
Kidney added he had no concern over Kearney's ability to finish the match, even though he has been restricted to just 140 minutes of rugby since last autumn.
Kidney revealed that blind-side flanker Stephen Ferris has played only a limited role in training since his comeback from a serious knee injury.
Kidney also said that hooker Jerry Flannery, who has been named on the bench, is to undergo a scan on a calf injury sustained in training.
The results of the scan are not expected until later on Wednesday at the earliest. Sean Cronin is on standby to fill the vacancy on the bench should Flannery fail to pull through.
Ireland team to play Australia:
Rob Kearney, Tommy Bowe, Brian O'Driscoll, Gordon D'Arcy, Keith Earls, Jonathan Sexton, Eoin Reddan; Cian Healy, Rory Best, Mike Ross, Donncha O'Callaghan, Paul O'Connell, Stephen Ferris, Sean O'Brien, Jamie Heaslip. Replacements: Jerry Flannery, Tom Court, Donnacha Ryan, Denis Leamy, Conor Murray, Ronan O'Gara, Andrew Trimble.