Favourite Australia will have to defy stall 12 in a field of 16 for Saturday's Investec Derby, the biggest assembled since 2008.
O'Brien's regally-bred colt has been a warm order for the blue riband Classic ever since beating the highly-regarded Free Eagle last September.
He was beaten on his return to action this season in the Qipco 2000 Guineas, but his pedigree suggests he can only improve for stepping up in trip.
The fact that two of the three horses to have run from the 2000 Guineas have subsequently won the Irish Guineas and the Dante and French Derby, in Kingman and The Grey Gatsby, also gives him solid claims.
As well as a big field and wide draw, though, Australia may also have to overcome softer ground than ideal with heavy showers forecast for Saturday morning.
Clerk of the course Andrew Cooper said: "We've had a lovely drying day today and the going is predominantly good with the odd softer patch, I'd say we're heading towards good for the Oaks.
"Potentially there is rain forecast for Saturday, but how much confidence you place is those forecasts I don't know."
Last year Joseph O'Brien chose to ride Battle Of Marengo only to see Ryan Moore partner stablemate Ruler Of The World to victory, but there was never any doubt he would be on Australia this time.
Moore is on Geoffrey Chaucer, while Seamie Heffernan rides Orchestra and Colm O'Donoghue is on Kingfisher.
Snow Sky will not run after failing to adequately recover from a minor setback.
The Michael Stoute-trained colt entered the Classic picture when winning the Lingfield Derby Trial, but it emerged on Wednesday morning that he had banged a joint.
Stoute and Teddy Grimthorpe, racing manger for owner Khalid Abdullah, had been hopeful of Snow Sky recovering in time to run but confirmed he will miss the race and head to Royal Ascot instead.
A statement from Grimthorpe read: "Michael Stoute reported that the colt was not quite 100 per cent this morning so will not be declared for the Investec Derby. He could now go to the King Edward VII Stakes at Royal Ascot."
The first three home in the Derrinstown all take their chances.
John Oxx's Ebanoran was first past the post that day but Dermot Weld's Fascinating Rock was awarded the race by the stewards after the pair got very close in the finish.
Oxx has only had three runners in the Derby, but has won it twice.
He said: "I can't really make any comparisons with my two Derby winners (Sinndar and Sea The Stars) as it is too early - Ebanoran has not won a significant race yet. He is quite an idle worker at home, like Sinndar, so doesn't give us too many clues about the extent of his ability."
Roger Varian's Kingston Hill is the only previous Group One winner in the race having landed the Racing Post Trophy last year and he heads the home team having been a popular choice in recent days, although he needs to overcome a tricky inside draw in stall two.
"Kingston Hill looks very well and we haven't had a hitch with him since the Guineas - I am anticipating plenty of improvement for that run," said Varian.
"He has got a good head on him so he should handle the excitements of Derby Day.
"Like the weather, the draw is out of our hands so there is no point in worrying about him getting stall two.
A mile and a half is a long way for them to sort themselves out and we will just have to hope that he gets a slice of luck at the right time."
Dante runner-up was Peter Chapple-Hyam's Arod while Saeed bin Suroor's True Story was third, with the pair aiming to emulate Workforce who was beaten in the Dante before winning the Derby.
John Gosden runs two in the unbeaten Western Hymn and the supplemented Romsdal.
Sudden Wonder and Pinzolo are the first Derby runners for Charlie Appleby. The latter will be ridden by James Doyle after , who was due to be on Snow Sky.
Our Channel represents William Haggas, Red Galileo is a first Derby ride for Oisin Murphy and Impulsive Moment runs for Andrew Balding.