Having spent almost two decades working for Gai Waterhouse, Steve O'Halloran admits it feels a little surreal to be saddling a rival runner in a $2 million race.

O'Halloran was among Waterhouse's go-to trackwork riders for many years, later working as a senior foreman and travelling with her horses to interstate carnivals, even taking Wandjina to Royal Ascot for the 2015 Diamond Jubilee Stakes.

Now a trainer in his own right, O'Halloran will be chasing by far his biggest payday with filly Nymphadora when she contests Saturday's $2 million Inglis Millennium (1100m) at Randwick.

"I spent seventeen years with Gai and I think out of her seven Golden Slipper winners, I was closely involved with six of them," O'Halloran said.

"I was educated by Gai and she taught me when to push the button with them and when to back off.

"There is no better two-year-old school than Gai's, she's got two in the race, so I am tickling myself a bit to be there."

Waterhouse and co-trainer Adrian Bott have two of the top Inglis Millennium fancies in $3 favourite Fully Lit and $6.50 chance Trunk, while Nymphadora is $61.

While O'Halloran's charge will have to overcome an awkward gate in barrier 14, those odds might be inflated if she can replicate her effort from the Inglis Nursery (1000m).

Shunted back to last after copping an early check, Nymphadora motored home late to finish an outstanding third, beaten less than 1-1/2 lengths by winner Odinson, who was a $4.80 second pick for the Millennium on Wednesday.

"I watched the head-on (replay) and the horse on her outside came right across her and put her out of the race, but her last 200 was unbelievable," O'Halloran said.

"Even Tom (Sherry) was taken aback by her turn of foot.

"She hasn't drawn well but last year second and fourth came down the outside so I'm hoping, especially with the track being a bit rain-affected, it might be the same again."

A $30,000 Classic Sale buy, Nymphadora is the second cheapest horse contesting the Inglis Millennium.

Although compact, the filly's calm demeanour caught O'Halloran's attention and he was surprised to acquire her so cheaply.

"Her attitude and the way she paraded was great," he said.

"I didn't think I'd get her for what we did, I had to look to make sure she had four legs."

While Nymphadora is a Millennium outsider, she has pleased O'Halloran with her build up, topping off preparations with a strong trackwork gallop at Hawkesbury this week.

He is heading to Randwick, if not confident, then at least optimistic.

"Being a two-year-old race, everybody probably thinks they can win," O'Halloran said.

"But on the way she has been running and going, if she can run top-five we will be over the moon.

"She has already paid back what she cost so anything else will be a bonus."