O'Brien in the dock for vicious punch
He will have to explain his vicious punch against French lock Pascal Pape.
The blow to the stomach came just 23 seconds into Ireland's 24-9 Pool D win over France in Cardiff at the weekend.
Pape needed treatment and furious French coach Philippe Saint-Andre labelled it on Monday as an "assault".
O'Brien, who was named man of the match, was cited by citing commissioner Dougie Hunter for a law that "deals with striking," said an official statement.
Marcelo Bosch of Argentina – who play Ireland in a quarterfinal on Sunday – has also been cited for a 'spear tackle' during his country's Pool C 64-19 victory over Namibia on Sunday.
Bosch had only been on the pitch a few minutes when he was sent to the sin bin for the lifting tackle – which sees an opponent fall head first to the ground – on Theuns Kotze.
O'Brien was not punished at the time, but his punch has become an Internet sensation.
Pape fell to his knees and needed two minutes of treatment before carrying on.
"It was an assault after 23 seconds of the game," said Saint-Andre who slammed match officials for not seeing the blow.
Saint-Andre questioned why referee Nigel Owens and the Television Match Official did not act straight away.
"The players have an idea as to why, I have an idea and I imagine you the press also have an idea.
"I hope that next time the TMO will really do his job.
"Since the beginning of the competition, on whatever matter is concerned, try or injury, we return to a review of the images.
"In this case they do no such thing."
Ireland's team management, preparing for a quarterfinal against Argentina with a growing injury list, had been nervously waiting to see whether Hunter would refer the case to a judicial officer.
"We're all aware of the incident, obviously," Ireland team manager Michael Kearney said before the official notice was released.
Coach Joe Schmidt had argued after Sunday's match that O'Brien had not landed a "closed fist" punch on Pape.
World Rugby has however been keen to clamp down on violent play.
AFP
