The 8-year-old son of Footstepsinthesand beat favourite and Makfi Challenge Stakes winner I Do by half a neck.
O'Sullivan co-trains Pure Champion with Andrew Scott who does 80 per cent of his trackwork rides.
"It's a lot harder as a trainer because you always have to take the horse back home, win or lose," said O'Sullivan, a New Zealand Racing Hall of Famer who had 2479 career wins as a jockey but agonisingly never won the Melbourne Cup. "As a jockey, you just walk away [after a race]," he said, emphasising horses require more time investment in training.
O'Sullivan said his stable wasn't really confident of winning on Saturday with Pure Champion, a group three winner in his native Ireland and twice winner in Hong Kong at the same level before arriving in New Zealand in winter.
"We hoped he'd run well and the horse was in good condition but you're never overly confident."
O'Sullivan, who owns the stallion with parents Marie and Dave, brother Paul and wife Bridgette, had instructed jockey Craig Grylls to surge ahead and hold his position early.
"I left it up to him from there on and just told him don't get shuffled back to the first 400 metres."
Grylls said the race followed the script he and the co-trainers had written.
"I stepped in, sitting in the rail and at the turn I steered him into the clear and he did the rest pretty much himself from there," the 24-year-old Matamata jockey said.
Grylls, who claimed his seventh group one win, including three this year, said the win was up there with the rest.
He had good vibes after racing Pure Champion on debut to third place in the Makfi Challenge on August 30.
"His work was really good. I was really confident because no one was really picking him [to win]," he said, believing the group one Livamol Classic here on October 4 as the final of the Bay carnival trilogy, would be ideal for the stallion.
Asked if Pure Champion would return, O'Sullivan replied: "Maybe, he'll come back in two weeks. Maybe."
I Do trainer Allan Sharrock was in a philosophical mood although the No 15 barrier didn't do his grey mare any favours as the field had to finish in a strong headwind in the final straight home.
"She's tried her heart out and I'm very proud of her.
"She did a bit of work very early and finished off the race well," Sharrock said.
He saluted Pure Champion, agreeing he had an inside draw but adamant no one could take anything away from the winner.
The 7-year-old I Do will line up at the fillies and mares race in Hawera.
Soriano finished third, three-quarter length back, with Sacred Star next in on a similar length.
O'Sullivan and Scott-trained Whosyourmaster won the last 1200m race, The Winning Edge Cup, with Hayden Tinsley in the saddle.
Predictably the group three HB Breeders Gold Trail Stakes went to the Stephen McKee-trained O'Marilyn and Sam Spratt.
Addictive Habit, with Bay interests, clinched the 2000m Livamol Classic Prelude and looks a prospect for the Spring Classic.
HB Racing CEO Jason Fleming said the poor turnout was realistic with the Hawke's Bay Magpies defending the Ranfurly Shield against Bay of Plenty.
"Rugby is a fantastic thing for Hawke's Bay and certainly today the emphasis was on that so we have to take it with some salt and tequila and move on."
Fleming said the crowd was relatively good and the turnover and investment was okay at a first-class race day.
TV rights made it difficult to reschedule events.
Fleming said their focus and energy would now be channelled into the Classic "grand final".
