The Galileo colt has been all the rage for the premier Classic since his close-up third in the 2000 Guineas at Newmarket on May 3.
However, Australia has not yet been tested in demanding conditions, with his trainer hoping the recent wet spell quickly recedes.
O’Brien said: “We wouldn’t like the ground to be soft.
“He’s a beautiful moving horse, everybody saw the class he showed at Newmarket.
“Obviously soft ground wouldn’t be ideal, but we’ll have to wait and see and hope it’s not.”
O’Brien confirmed at the ’Breakfast with the Stars’ morning at the Surrey venue yesterday that Australia may be accompanied by Geoffrey Chaucer, Kingfisher and Orchestra.
Joseph O’Brien, the trainer’s son, is set to partner Australia, while Ryan Moore, who won the Irish 1,000 Guineas last weekend aboard the stable’s Marvellous, is poised to get the leg-up on Geoffrey Chaucer.
The latter has this week been well backed in the Derby market after rumours circulated of the colt having beaten Australia in a gallop at Ballydoyle.
O’Brien said: “I don’t know where these (rumours) come from.
“They have obviously always been on different work regimes and both had different races last time.
“I don’t know where it came from. I didn’t see it if it was, maybe it happened, but I didn’t see it.”
Frankie Dettori admitted yesterday he was on the lookout for a ride in the Derby.
When asked if he would consider the Italian, O’Brien said: “It would be very hard to get any better than Frankie.
“Everyone knows how good he is, he’s a special man.”
Connections of Arod warned that the improving colt will not run in the Derby if the ground went soft.
Michael Stoute, by contrast, would be unfazed if the ground was testing for Snow Sky.
The Khalid Abdullah-owned colt excelled in the Lingfield Derby Trial on May 10 and is already proven in demanding conditions.
The Freemason Lodge handler said: “He’s entitled to go (for the Derby) after his Lingfield performance, we’re hopeful.
“It’s still a long time away and Epsom drains very well.
“He won on soft ground at Salisbury and I’m not concerned about ground conditions.”
Romsdal will work at Newmarket today to determine whether or not he will be supplemented for the Derby.
Stablemate Western Hymn, who defeated Snow Sky in April and subsequently won Sandown’s Classic Trial, is very much on course for Epsom after he enjoyed a nice workout under William Buick.
He said: “Western Hymn enjoyed coming here today. He handled the hill, switched his legs very well and his head carriage is better.”
Kingston Hill gained plenty of admiring glances from work-watchers as last season’s Racing Post Trophy winner pulled a couple of lengths clear of two lead-horses in a 10-furlong stretch.
Meanwhile, Shamkala is more likely to remain on home soil than travel from France to contest the Investec Oaks at Epsom on Friday week.
The Sea The Stars filly, trained in Chantilly by Alain de Royer-Dupre, has been prominent in the betting for the Classic since she coolly won the Group Three Prix Cleopatre at Saint-Cloud on May 17.
Shamkala still holds an entry in the Epsom Oaks and she has been prominent in the ante-post markets, but connections of the Aga Khan-owned three-year-old are instead keen to run in her the French equivalent at Chantilly a fortnight later.
Aidan O’Brien could have up to five runners at Epsom, staged this year in memory of Henry Cecil.
Ballydoyle’s battalion will be spearheaded by Marvellous, winner of the Irish 1,000 Guineas at the Curragh last Sunday.
Stablemate Bracelet was well beaten in the 1000 Guineas at Newmarket earlier this month but she, too, could feature.
O’Brien said: “Bracelet is a possible. There’s also Tapestry, Marvellous and I suppose the filly (Dazzling) that ran in the trial in Navan.
“We’d be looking at four or five possibles at the moment.”
Irish interests are polarised by Tarfasha, accomplished winner of the Group Three Blue Wind Stakes at Naas.
Trainer Dermot Weld will, though, consider other alternatives if the ground further deteriorates.