Good fortune had become a familiar companion since his shrewd purchase of Killula Chief out of a selling contest - the gelding's seven subsequent successes from nine outings even included the rare distinction of winning a race after falling and being remounted. But at the seventh fence in the Sun Alliance Chase, Killula Chief's luck ran out.
Five months on, O'Shea has started the new jumps season with a string of 25, almost twice the size of last year's, and, on his 32nd birthday nine days ago, a double at Hexham. Yet the loss of his finest performer remains painfully fresh in his mind.
'The sorrowful part is that he was killed because he was too good a jumper,' the trainer recalls. 'He passed four horses in the air, he just overjumped. I'll never forget it. You just don't realise how close you can get to them.'
There are dozens of ambitious young trainers with single-figure strings who deserve a horse like Killula Chief, to grab some headlines and propel them a crucial couple of steps further into the big time. A yard may be full, but without at least one good animal it can only tread water. Or sink.
Few of them get one chance, even fewer a second, so it is a tribute to O'Shea's talent and determination that he is currently plotting a campaign for Go Ballistic, who finished third in the Festival Bumper the day after Killula Chief's fatal fall.
With speed which belies the bloodlines of a top chaser, Go Ballistic has as much potential as any novice hurdler who will see a track this year. And until last week, the man doing the plotting was David Nicholson, the champion jumps trainer.
If an owner is prepared to remove such a promising performer from Nicholson's care and place it into O'Shea's, then the man who left County Carlow as a 17-year-old to try his luck as an apprentice jockey on The Curragh has clearly found his true calling.
'I was far from being a good rider,' he says, 'I had about 40 rides with about half a dozen seconds, a couple of which should probably have won.' Opportunities were restricted by his peers. 'I served my apprenticeship with Liam Browne, and the others there at the time were Michael Kinane, Stephen Craine, Tommy Carmody and Mark Dwyer.'
With typical realism, O'Shea quickly decided that training offered a greater chance of advancement and he took out his first licence in Ireland at the age of 24, moving to Britain shortly afterwards. 'I started off not knowing anyone, and the first year I was just wandering around trying to find owners.' He now has several loyal supporters (Killula Chief's owner has bought three horses to replace him) and his string is quickly expanding.
Honesty - rated a trainer's most important quality in a recent survey of owners - may have something to do with it. O'Shea also has an ability to pluck talented performers from the murky waters of claimers and sellers.
Apart from Killula Chief, he had also recognised the promise of Land Afar, last year's William Hill Handicap Hurdle winner. Land Afar was bought out of a claimer but - another painful memory - the gelding was moved to John Webber's yard before O'Shea had a chance to justify his judgement. This term, Rainbow Walk, who cost 15,000gns out of a Wolverhampton claimer, could be the one to put the record straight.
With his willingness to sift through several layers of the form book in search of cheap but capable new recruits, it is little surprise that the trainers O'Shea most admires are Martin Pipe and John White.
Championships and Grand National winners may yet be some way distant, but even O'Shea will admit that the momentum is beginning to build. 'It's always a struggle, but it's definitely a bit easier than it was two years ago. Killula Chief had to be the turning point, but you never realise yourself that you're climbing the ladder.'
The arrival of Go Ballistic will focus attention on his yard, but O'Shea will gladly accept it. 'One thing I've always had is great belief in myself,' he says. 'I'm sure the road won't be smooth from here on, but I'm still determined that I'll get there.'
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