Aidan O'Brien will today send a four-strong team into the Investec Derby at Epsom, with bookmakers Coral offering odds of 33-1 that he saddles the first three horses home in the premier Classic.

The outright favourite is Australia, while Ryan Moore's mount Geoffrey Chaucer and Chester Vase winner Orchestra also harbour reasonable claims on victory, and it would not be a complete surprise were those three to dominate the field. O'Brien also runs Kingfisher.

However, despite the likelihood of the ground quickening up yesterday as a result of the sunny weather, the prospect of heavy showers following today made it much more difficult to identify what conditions will be like for the big race.

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"The presence of Australia in this year's Derby will ensure turnover is healthy, as for all those punters who want to be with the favourite, there will be plenty looking to oppose him," said David Stevens of Coral, with the bookmakers confirming that they expect a turnover of £20m. "The market leader has been easy to back all week and his price could yet drift further, particularly if Epsom is hit by the thunderstorms the forecasters are predicting."

O'Brien has made little secret of the high regard in which he holds Australia, with the trainer having described him previously as the very best Flat horse he has worked with. "Australia is uncomplicated, relaxed and has speed; exactly what you want in a racehorse," he said. "He's very natural and takes no training. He has a great mind and settles very well."

Such an contented account of his chief runner did not diminish the ambitions O'Brien has for his other horses in the field. "I was very happy with the way that Orchestra won at Chester. If anyone had seen him there, they'd have seen that he had a bit of a tummy on him and he'll come on for that," added the trainer.

"Kingfisher won well at Chester and is very straightforward horse. Nothing went right for Geoffrey Chaucer in the Derrinstown at Leopardstown as it was a very messy race with a slow pace. Joseph [O'Brien, the jockey] had to go up the inside from where he was drawn."

The only Group One winner in the race is Roger Varian's Kingston Hill, although he was some way behind Australia in the 2000 Guineas. The horse would likely benefit from any rainfall. "He deserves to be there. He's a Group One winner and I have no doubt you'll see a better horse now he steps up in trip," said Varian.

One runner that will be impeded by the rain is Saeed bin Suroor's True Story. The mount of Kieren Fallon, the horse was third in the Dante behind subsequent French Derby winner The Grey Gatsby.

"The Dante form is good," said Bin Suroor. "Since York he has worked twice with Kieren and worked really well. I'm really happy with him, but the key is we don't want it too soft. The horse is well, physically we are happy now, he's improved a lot and I think a mile-and-a-half will be really good for him. Good ground or good to soft will be perfect."

One place in front of True Story at York was Peter Chapple-Hyam's Arod, representing a trainer with two wins in the Classic. "Arod has been in really good form since the Dante - we are really happy with him. We think he has improved and he has definitely improved physically," Chapple-Hyam added.

Dermot Weld has won races all over the world but has yet to triumph in the Derby, with Casual Conquest's third place finish behind New Approach in 2008 the closest he has come. His contender this year is Fascinating Rock, who won the Ballysax Stakes and was awarded the Derrinstown Derby Trial in the stewards' room.

"We hope he will stay. The mental attitude of the horse will help - he's a lovely, big, relaxed horse," said Weld, who was second in the Oaks with Tarfasha.

"He got a mile-and-a-quarter at Navan going away, he was coming home to some tune at Leopardstown. He gives us every indication that he will stay, but that is the one doubt about it."

Fascinating Rock won the Derrinstown at the expense of John Oxx's Ebanoran. The Currabeg handler has fewer concerns over the trip. "The longer trip is a new test and Epsom is a very demanding track," said Oxx, who has won with two of his three Derby runners to date. "You cannot be certain but he has so much stamina on the dam's side of his pedigree that there must be a good chance he will stay the extra distance."

It seems a long time since John Gosden savoured Derby success with Benny The Dip in 1997 and he fields the only unbeaten runner in the race, Western Hymn, having landed his first Oaks with Taghrooda.

"He is a young horse and has only had three races," said Gosden, who has also supplemented Romsdal, beaten a nose by Orchestra in the Chester Vase.

"Western Hymn has a lot of ability and I am very pleased with the way he has trained this year. He is a very solid and legitimate horse to run in the race."