O'Brien has been handed a one-week ban after he was found guilty of striking French second-row Pascal Papé (pr: Pa-pay) during last Sunday's Pool D match.
The Independent Disciplinary Officer, ruled that O'Brien's offence had been at the low-end of the punishment scale, with his initial two-week punishment halved to just one week because of the player's previous disciplinary record.
Meanwhile, the Irish squad will return to training this morning as they attempt to recover from the loss of Paul O'Connell.
Mike McCarthy will be involved in the work-out in Cardiff after flying into the camp last night.
An update is also expected on the fitness of Johnny Sexton, who looks set to be given the all-clear from a groin injury.
O'Connell joins Peter O'Mahony and Jared Payne as the players that have been forced-out of the squad over the past week.
Head coach Joe Schmidt says that all three players are huge losses.
