As the son of the world's most prolific trainer, Joseph O'Brien has always expected he would follow his father Aidan.

On Tuesday, the 24-year-old Irishman did something his father has not, winning the Melbourne Cup with Rekindling for retired Australian businessman Lloyd Williams.

And while he was born to train racehorses, that was on hold when Joseph O'Brien unexpectedly became a champion jockey.

"I always wanted to train," O'Brien said.

"It was what I was always going to do.

"When I became a jockey I intended riding 20 winners which you can do and then become an amateur rider in Ireland.

"I was a bit more successful than I thought I would be."

That is a modest understatement with O'Brien twice Ireland's champion jockey and a prolific Group One winner.

Among his major European victories are two Epsom Derbys on Camelot (2012) and Australia (2014), and many wins at Royal Ascot including the 2012 Prince Of Wales's Stakes on dual Cox Plate winner, Australian horse So You Think.

He also won the 2011 Breeders' Cup Turf on St Nicholas Abbey and the Dubai Sheema Classic on the same horse two years later.

After battling his weight for many years, O'Brien called time on his riding career in March last year and set up stables about an hour from his father's famous Ballydoyle establishment.

He claimed his first Group One win in last year's Moyglare Stakes with Intricately.

He has been anointed by Williams to take over from his father as the world's best and his Cup win will go a long way to that prediction.

"I can hardly believe it," he said.

"I have spoken to Dad on the phone. It hasn't really sunk in yet."