The Galileo colt has been all the rage for the premier Classic since his closely contested third place 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.
"He's a beautiful moving horse," said O'Brien yesterday. "Everybody saw the class he showed at Newmarket. Soft ground wouldn't be ideal; we'll have to wait and hope it's not."
O'Brien added 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 1000 Guineas last weekend aboard the stable's Marvellous, is poised to get the leg-up on Geoffrey Chaucer.
Also aiming for a Derby mount is jockey Frankie Dettori and when asked if he would consider the Italian, O'Brien said: "It would be very hard to get any better than Frankie. He's a special man."
The possibility of soggy underhoof conditions is also worrying connections of improving colt Arod, trained by Peter Chapple-Hyam, which made up lots of late ground at York in his third career start to finish second in the Dante.
Jockey Jamie Spencer said: "It looks like Arod is going to run in the Derby. He needs fast ground, so watch this space."
Those thoughts were echoed by Arod's owner, Sheikh Fahad, who said: "He needs good ground. Good to soft maybe, but not worse."
Sir 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.
Today at Newmarket, Romsdal will be looked at to determine whether he will be supplemented for the Derby.
Beaten just a nose by Orchestra in the Chester Vase, the Princess Haya of Jordan-owned colt could instead run at Royal Ascot.
Trainer John Gosden said: "He'll work in Newmarket and see what the owners want to do. He's in the King Edward at Royal Ascot, so it's up to them."