The bulldozing Leinster flanker is currently Ireland's outstanding performer with his destructive ball carrying seeing him crowned European player of the year.
Having consistently left a trail of defenders in his wake since nailing down his place in the team during this year's RBS 6 Nations, he has become a marked man.
The 24-year-old knows he will come under close scrutiny from Ireland's Pool C rivals, but will deal with the attention in his own unique way.
"Looking for space is important to ball-carrying, as is evasion and getting your legs pumping when you hit contact," he said.
"There are lots of ball carriers in this World Cup and I'm sure we'll all be marked closely.
"That happened to me towards the end of the Six Nations. In the third game there were more lads coming towards me.
"But that creates opportunities for those around me - if we're smart enough to take advantage.
"I'm sure opponents will mark me up pretty well. That's something you have to deal with. If there aren't any holes, I'll just have to make one."
O'Brien is likely to start at openside when Ireland announce their team for Sunday's opener against the USA in the early hours of Friday morning.
The serious knee injury sustained by David Wallace against England last month has left a vacancy in the number seven jersey.
O'Brien has just recovered from his own knee problem and the management may yet decide not to risk him in a match Ireland are sure to win.
But if asked to fill Wallace's shoes, he is confident he will be able to adjust him game.
"I'm comfortable in all three positions in the back row. It doesn't bother me where we play," he said.
"I'm pretty happy at seven, I've played plenty of games for Leinster there.
"Playing at seven does reduce your capacity to carry a bit. You have to look after the ruck and link up with the backs more.
"At six you have more of a free role and you can come onto the ball a little harder. I have to adapt to that, but it's only a minor tweak to my game."
O'Brien's ball-carrying prowess could see him become one of the stars of the World Cup and Ireland's most destructive player is eager to prove himself.
"When you're on the world stage playing against the best, it's time to test yourself out and see where you are in your career," he said.
"I'll know a lot more about myself at the end of this tournament. Absolutely I feel ready to rise to the occasion.
"I'm relaxed and confident about coming up against anyone."
Ireland lost all four of their warm-up internationals in the build up to New Zealand and need a statement of intent when they face the USA in New Plymouth.
"We were disappointed with what happened last month. We played some nice rugby at times, but were sloppy," said O'Brien.
"The lads are out to prove a point against the USA. If we do the basics well and stay focused we will do well."
Should Ireland decide to give O'Brien another week of recuperation, Shane Jennings will start at openside with Stephen Ferris and Jamie Heaslip completing the back row.
Cian Healy is doubtful because of his eye injury so Tony Buckley is likely to slot in at loosehead with Tom Court's versatility keeping him on the bench.
Geordan Murphy is on standby should Rob Kearney be rested for Australia, while Eoin Reddan and Connor Murray are battling it out for the scrum-half duties.
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