His comeback is so close that he can practically taste it. Hours in rehabilitation, painstaking meticulous pokes, prods and drills with no end result. For those who are born to run, injury is a trying part of life. And it is in those hours when you take stock and refocus the mind for the day when you can run out in front of roaring crowds.
And speaking to O'Brien as he enters the closing stages of his rehabilitation after corrective hip cartilage surgery which was undertaken in June, it's clear that he can't wait to pull on the blue jersey once more.
"You know, this isn't the first time that I've been injured in my career," the 25-year-old revealed this week.
"So on one side of things I'm used to the process and you just get on with your recovery. But as you get older and you miss out on big games and playing with your friends in front of huge crowds, it hurts.
"At the moment I'm feeling fit and strong and the comeback isn't that far away. All things going well I hope to make either the Glasgow (away) game or the Zebre game in the RDS the following week. It's a good feeling being able to see the forest for the trees.
"But I'd be lying if I said that these last few days and weeks have been easy. They can really drag because all you want to do is get out there and play.
"You nearly do more work in rehab than when you're fully fit. You could be in early in the morning with the S and C (strength and conditioning) guys, or the physio, and then there could be weights in the afternoon."
The fact that he has been joined on the sidelines at various stages over the last few months by a huge number of colleagues, a dozen or so for the most part, has been a help.
"There has been a core of lads who have been injured for a long time and that has been a blow for the team to deal with because at times we have been really stretched, resource wise.
"We're working off similar schedules, so at least you're not working away on your own because that can be demoralising and mentally tough at times.
"But as that first game gets closer, the excitement builds. It gave all of us a lift to see the likes of Rhys (Ruddock) get back on the field in recent weeks and for a few of us, our turn is coming up shortly, hopefully."
It is widely known that O'Brien hails from proud farming stock. But it wasn't just to the land that he turned to keep him busy over recent times.
"There's always work to be done on the farm, and that's a lot of hard work but a labour of love for me.
"I'm back in college in UCD completing a degree in sports management, which has been really enjoyable. It's a lot of work but I know that it will stand to me in time, so that takes up around three days a week.
"Obviously then on a Tuesday and Friday night I'm down coaching in Tullow RFC and that is really important to me.
"There's a great club man called Victor Doyle who helps out with the forwards and it's a real challenge trying to get the best out of players, but one I enjoy. The lads are super keen and there's a great camaraderie in the group and with the injury I've been able to spend a bit more time learning my trade there.
"We played County Carlow in a derby match last weekend and there's a great buzz around the area on weeks like that. That kind of involvement keeps me connected with my roots and it's hugely important. So, even though I'm not playing, I'm far from bored!"
Tomorrow afternoon's game against Fiji will evoke special memories for O'Brien, for it was on a blustery November evening three years ago against tomorrow's opponents in the RDS that he came off the bench to make his senior bow (alongside provincial team-mate Jonathan Sexton) in a 41-6 victory.
Ask him to temporarily pause and step back in time to that game and the 2012 European Player of the Year paints a vivid picture about his feelings as he stepped up to hear Amhrán na bhFiann.
"Being Irish and playing for Ireland means so much.
"It's hard to describe the feeling of standing up for the anthems for the first time and trying to stay focused on the game.
"Lining up on the pitch, I remember my mind wandering to my family and friends who were in the crowd and all those who had helped me get there.
"If I'm honest, I would still get a bit choked up trying to describe it, but it's something that will stay with me forever," the wing forward reflected.
"Debuts are big moments in players' careers and that point struck a chord with me when James Tracy came off the bench for his first appearance a few weeks back against the Ospreys.
"You know, we didn't get the result we wanted that day, but it was a great learning curve for the young team we had out there. It will give them a real taste for top class rugby."
O'Brien has had the taste and it's clear that he also has the hunger.
And the time when he can finally make a tangible physical difference is imminent.