The 35-year-old handler endured a tough time during the first eight months of this year with fancied and well-touted runners failing to come up to scratch in the big races.
But two Group One successes in the space of three days four weeks ago with Ad Valorem (Middle Park Stakes) and Oratorio (Grand Criterium/Prix Lagadere) marked a noticeable turnaround in fortunes for the stable which was backed up by near-misses in the likes of the Fillies' Mile, Royal Lodge and Dewhurst.
And so O'Brien arrives in Texas with realistic hopes that he might be able to improve upon his tally of three Breeders' Cup successes.
Arguably chief among his prospects is Powerscourt, who along with jockey Jamie Spencer will bid to put the nightmare of his last American experience behind him when he lines up in the Turf.
First past the post in the Arlington Million in August, the four-year-old was demoted to fourth place for causing interference when Spencer's mount drifted across rivals.
"Obviously he is a very good horse who handles fast ground but being by Sadler's Wells, he should be adaptable to softer ground," said O'Brien.
"I always felt that maybe we haven't got the best out of him. He's always a horse who has been very highly thought of, he's just little bit tricky to have spot-on as he can come and go very quickly.
"The step up to a mile and a half should suit him well and in an ideal world you would like see him sitting in the first five or six.
"His last piece of work before he came was exceptional and hopefully the injury he picked up in his last race at Leopardstown hasn't taken too much out of him."
Yesterday returns to the Breeders' Cup Filly & Mare Turf having finished third to Islington in last year's renewal.
"I think she has come forward since France," reported O'Brien. "She is a very intelligent filly and although she handles fast ground I feel that sometimes she is a bit careful on it.
"Sometimes she can take a couple of strides to quicken up so hopefully the softer surface will help her.
"She is very lucky to be here at all really, having nearly died of colitis during the summer. We're not even sure if she's fully recovered, but if she has then she is capable of running very well."
Exasperating character Antonius Pius has been another member of O'Brien's string on whom Spencer has not always excelled this year.
But the unique conditions of the Mile could just suit Antonius Pius, about whom O'Brien said: "He's probably a horse that you haven't seen the best of yet.
"Jamie will have to ride him for luck and what you'd love for him is a fast-run race.
"He's a very tricky horse to ride but maybe he just hasn't grown up yet - he's not physically immature but he might just be mentally immature.
"He won't be an easy ride but he is capable of anything."
The O'Brien contingent is completed by his two two-year-olds, Mona Lisa (Juvenile Fillies) and Scandinavia (Juvenile).
"We are taking a bit of a chance with them both but they are bred to handle the dirt and they both have enough tactical speed to make sure they don't get behind," said O'Brien.
"Scandinavia is a horse very much on the upgrade and it's just a question of whether this comes too soon for him really.
"When you bring horses here you just have to hope that they acclimatise and adapt in time for the racing.
"All five will canter on the dirt tomorrow and then those that are racing on the Turf will have a stretch on there on Friday so they get the feel for it.
"We're just hoping for good runs from them all. You can't come here expecting to win, just do your best and then you never know.
"At least there's light at the end of the tunnel now. Earlier this season things were looking a lot worse but you always learn from your mistakes you make decisions one year that you wouldn't make the next.
"Jamie's had a tough time this year but he's got a huge future ahead of him. He's learnt a lot about racecourses, about me and about everything.
"We talk everything through and neither have us have ever held back on anything. Things haven't always worked out for him this year but I'm sure we can put things right."