"We were a score away from them and we played very poorly. So you'd have to be confident, if we get it right, that we can beat them," he said.
"I suppose we'll have to start a lot quicker. We were playing catch up after 25 minutes and it's hard to play catch up against any international side. So we'll have to start well and sort out our ruck probably as well.
"In the first 10 or 15 minutes we were kind of like, 'should we go in and compete or should we not?'. There was a bit of indecision there by a lot of us.
"If we sort out a few things like that, a few of the basic things and hold on to the ball, we should be good."
The newly-named Guinness Rugby Writers of Ireland Player of the Year and reigning ERC European Player of the Year also brushed off concerns about Ireland's lack of tries in their opening two warm-up matches.
"I wouldn't t say it's a worry at all. If you look at the last couple of games we played against France, we've outscored them on the try sheet a couple of times," explained the 24-year-old forward.
"It's just about getting our attack right and if we hold on to the ball, the tries will come."
The Carlow native took just three-and-a-half weeks off at the end of the season before reporting back for international duty and he was happy to get his first 80 minutes under his belt in France.
But after trailing 13-3 at half-time, O'Brien says he was frustrated that Ireland did not manage to overhaul a tiring French side in the second half.
"We spoke about that - that they might tire a little bit and we'd have to keep going. They probably did at times but we just didn't have that killer instinct about us out there.
"We kept the ball a lot better in the second half, we sorted out the ruck a little bit and we started putting a few phases together. But we dropped a couple of balls - I dropped one or two myself - and that's not on really."
After working hard over the summer, O'Brien will now concentrate on getting battle ready for next month's World Cup in New Zealand.
"I'm feeling strong and fast. I'm happy enough with the way it's gone so far and a couple of games now and I'll get match fit," he added.
"You can do all the running and all the pre-season that you like, but it's actually getting out there and playing the games that gets you properly fit. Match fitness is one thing and running around for a couple of hours is another thing."