The father and son team joined forces for the start of the current season with a mix of proven and untried horses in their Matamata stable.
McKay senior has been training for more than three decades and most notably guided the large majority of the career of the multiple Group One winner Alamosa before his retirement to stud.
He shared in the ownership of the son of O'Reilly with wife Kim and they also raced the inaugural Karaka Million (1200m) winner Vincent Mangano.
Champion apprentice in 2015-16 season, he has a career tally of 257 domestic winners, nine of them at Group or Listed level, and recent injuries prompted him to consider training as a long-term option, while he enjoyed a stint with successful Victorian trainer Danny O'Brien last year.
“It's something I looked into for the future while I've been riding and I didn't have the best of seasons last season,” McKay said.
“I talked to Dad about it and he offered me the partnership, I took it and I'm now really looking forward to see what the future holds.”
McKay, whose brother Jacob also formerly trained with his father before a stint on his own, has no immediate plans to bring the curtain down on his career as a jockey.
“I will continue to ride, I've got my dual licence, and will definitely ride for the rest of this season. I'll see how it goes and then possibly another season, but for the moment I'll do both,” he said.
“I had a fall and that put me out for a month and then I came back and it was pretty slow and at the end of the season I had surgery on my nose, which I broke in the fall.
“I was out for another three weeks with that as well so I had a couple of setbacks.
“I've been cleared to ride again and, depending on whether I get any outside rides in the next couple of weeks I'll ride, but otherwise I'm more than happy to wait for our team to fire up.”
The new training partnership is planning to keep its powder dry until the Hawke's Bay carnival.
“We haven't had a runner yet and we probably won't have anything race until the first day at Hastings,” Shaun McKay said.
“Sagunto is likely to go down there and maybe a couple more, but they are weather dependant at the moment.”
O'Reilly's son Sagunto has two past wins to his credit at the Hastings meeting and continued his progress toward a return to action with an outing over 1300m in an open heat at Tuesday's trial on the synthetic track at Cambridge.
“He was good and had a quiet trial under his own steam and he ran a nice second,” McKay said.
“We've got a nice team with a few of the older horses coming up well for the open handicap races and some nice three-year-olds as well so we're hoping for a good season.
“At the moment, we've got 12 in work and in the next month or so there will be more coming in. We'll be pretty busy and we don't want more than 18.”