The latest racehorse owner to dump jockey Damien Oliver says he merely made the best decision he could for his horse.

Terry Henderson, who operates the OTI ownership syndicate with former Test cricketer Simon O'Donnell, confirmed Oliver had been replaced by Corey Brown on his Caulfield Cup entrant My Quest For Peace.

Henderson's announcement coincided with the revelation on Sunday of a stewards investigation into allegations Oliver bet $10,000 on a rival horse in a race in which he was riding in 2010.

But Henderson would not directly link the two events.

"We've chosen the best rider for the race and that is Corey Brown," Henderson said.

My Quest For Peace is a 20-1 chance in Saturday's $2.5 million Caulfield Cup.

In another riding shuffle in the wake of the betting allegation, Oliver has also lost the ride on Green Moon, the second favourite for Saturday week's Cox Plate.

Green Moon's new rider is now his regular partner Craig Williams who gained a release from the favourite Pierro to get back onto the horse.

In reaction to the riding switch, and in reaction to Pierro suffering his first defeat in nine starts in Saturday's Caulfield Guineas, Pierro has drifted from $1.85 to $3.50 in Cox Plate betting.

As much as $5 is being offered about the colt on Betfair.

Williams said he had not instigated the riding switch, saying he had been approached by Green Moon's owner Lloyd Williams and then contacted Pierro's trainer Gai Waterhouse to seek a release from her colt.

"In the end I was in the position where I had the choice of two really good horses," Williams said on Monday.

He said one of the deciding factors in favour of Green Moon was Pierro's Cox Plate weight of 49.5kg.

"I had to weigh up a couple of options, one was the weight.

"I'm very, very comfortable with my decision."

Williams wouldn't comment on the Oliver issue or on allegations made last month that race fixing was rife in Victoria.

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