By now,
we have all seen the pictures and video footage.
Pascal Papé grabbed hold of O'Brien early on, in
Ireland's 24-9 win over France, and got a jab to the stomach in return.
It was reckless from the Irish flanker but Papé - perhaps taken unawares - made the most of it.
As for his potential citing, here's what Joe Schmidt had to say on the matter.
"I truly hope [a lengthy ban is not inevitable].
"He was being held at the time and he swung. I don't think he is looking directly at the player but it is not for me to determine. I also think that it's not a closed fist. I'd be hopeful but it is not a decision for me.
"It's hopefully not something that will impact on us, considering the dressing room we have at the moment.
"With the amount of injuries we have, we can ill-afford to lose another player. Particularly a player of Sean's experience and standing within the group."
The citing process
The Citing Commissioner, who is from a neutral country, has 36 hours from the moment the match ends to make a decision against a player. That means O'Brien may not hear about any proposed bans until before 7am on Tuesday morning.
If the commissioner needs more time to make a decision, he can ask for an extension from the Judicial Officer.
The likelihood is that were O'Brien to get cited, he would know by later tonight with an appeal taking place on Tuesday afternoon.
The commissioner - using various camera angles, TV feeds and Hawkeye - may cite O'Brien for an act of foul or illegal play contrary to Law 10.4 of the game.
IF O'Brien is cited, he is entitled to a hearing within 48 hours of the end of the match.
At an appeal, it is the Judicial Officer that hands down a ban - taken from World Rugby's Recommended Table of Sanctions - if he believes the player to be guilty.
In making that decision, the officer will take into account the seriousness of the offence, aggravating circumstances [ie: O'Brien being provoked] and 'off-field mitigating features', such as an admission of guilt, reconciliation with an injured player and past disciplinary record.
IF O'Brien is found guilty, he will be informed within 24 hours of his hearing and have 48 hours in which to appeal any ban or sanction.
Then, an appeal should 'where reasonably practicable' take place within 48 hours of it being lodged.
At this hypothetical stage, we are into Dublin GAA territory, meaning O'Brien's appeal could happen on the eve of the World Cup quarter final with Argentina.
For now, Irish supporters live in hope that O'Brien may get off with a warning.
