"He's Liam's ride," said the man who has acted as mentor while Cooper has developed from a raw 7lb claimer in Cumbria to become the man responsible for getting the best out of some of the most valuable jumpers in training.
The transition hasn't always been smooth - and some gossips still insist that O'Neill and AP McCoy is a marriage waiting to happen - but Cooper has repaid the faith of his boss both on the track and off it.
In the saddle he inspires some punters and infuriates others with his tendency to rely on swooping as late as possible.
And out of it he does a good impression of a one-club man who knows exactly which side his bread is buttered.
In an era when most top jockeys have a contacts book thick enough to choke a donkey, Cooper devotes himself almost exclusively to going where O'Neill needs him.
The spare ride on Marlborough in the King George at Kempton remains his sole outside mount from more than 180 rides this season, while despite his success in a string of big races it is far from unusual to see him mucking out with the rest of the staff at O'Neill's Jackdaws Castle yard.
Rightly or wrongly, the O'Neill stable still carries the reputation as among the shrewdest in the land when it comes to landing gambles.
But you wouldn't like to be the interrogator charged with prising any precious nuggets of information out of Cooper.
Playing squash against a mattress is the phrase which comes to mind when trying to glean anything more than the obvious, though he does drop his guard briefly when it comes to discussing the best chaser he has ever sat on.
"It was tough to miss the Tommy Whittle, but I was delighted with the way Keen Leader performed at Haydock and just watching him from home was as good as riding a winner in itself," he said.
"I've never been the sort to take things for granted, because this game can give you a lot of knockbacks, but he's a very good horse and I'm looking forward to riding him again."
O'Neill, who won the Gold Cup aboard Alverton in 1979 and Dawn Run in 1986, would love to train a winner of the great race and feels Cooper knows Keen Leader better than anyone.
"Liam has done loads of schooling on him and has worked hard to get where he is." he said. "Best Mate is a true champion, but we've always thought Keen Leader was a Gold Cup horse and if the ground is soft for him then so much the better."
Truckers Tavern will be in opposition again on Saturday on a track which connections feel will suit him ideally, while Sir Rembrandt is another likely opponent after his courageous second in the Welsh National last month.
But Keen Leader has the form and the scope to put them in the shade if his jumping passes the Cheltenham test.
Look down the list of top jump jockeys and it's clear that some star names are likely to be watching the Gold Cup on television this year. Richard Johnson and Ruby Walsh have no links with any of the leading contenders so far. Geraghty is also likely to be available, while McCoy could be in need of a mount if Therealbandit runs in the SunAlliance Chase instead.
But Cooper is the man in possession of the ride on Keen Leader. He doesn't intend giving it up without a fight.
