The world No. 1 sunk her tee shot on the 140-yard par-three eighth, the highlight of an impressive six-under-par round of 65 which also featured four birdies.
Remarkably, it was the first hole-in-one of 19-year-old Ko’s career — and the second of the day on the eighth after Lin Xiyu of China also achieved the feat — leaving her unsure of how to react.
“I would have loved to have done a dance or jump up and down but I did not want to look stupid if it was just close to the hole,” the New Zealander told reporters.
“It was the perfect shot at the perfect time and, to do that at the Olympics, it does not get any better than that. It puts the cherry on top.”
Ko started the day seven shots off the lead but will go into Saturday’s final round tied for second with Gerina Piller of the USA on nine under par, two behind Park.
China’s Shanshan Feng was a shot further back after matching Piller’s third-round 68.
“I had a (score of) six under in my head and to have done that is really cool,” Ko added.
“I did not expect the wind to be this strong, so it feels more like 10 under. I’ve put myself in a good position going into tomorrow and still have a chance to be standing on that podium.”
Park could not match her blistering form on Wednesday and Thursday, when she carded back-to-back rounds of 66.
Yet Friday’s 70 looked solid compared to playing partners Stacey Lewis of the USA and Great Britain’s Charley Hull, who were one and two shots behind her overnight.
Both faded away, with Hull carding 74 for a tie for fifth place on five under and Lewis dropping to equal eighth after a 76.
There were similar frustrations for Brooke Henderson of Canada, who started Friday tied for third with Hull but could only score a disappointing 75 to be one of seven players along with Lewis on four under.
China’s Lin, meanwhile, could not capitalize on her ace and finished the day in 46th place after a 74 left her seven over par.
The field of 59 players is seeking to win the first women’s golf event at the Olympics since the 1900 Paris Games.