The 23-year-old from Mystery Creek, Waikato, swept aside nerves and pressure to pocket the $40,000 winner's cheque aboard Dunstan Delphi, an eight-year-old mare bred by her father Jeff McVean.
She headed off a 19-strong field challenged by a technical course featuring fences of up to 1.6m in height.
She finished ahead of experienced Dannevirke competitor Maurice Beatson, who rode two horses, Zibbibo and Apollo van Evendael, to finish second equal with himself.
McVean, who last won the Olympic Cup in 2008 aboard Dunstan Forrest II, was left stunned despite enjoying a stellar season in which she has won a series of prominent titles.
"It is going to be hard to keep a horse as good as Delphi. She is just unbelievable," McVean said.
In the three-star eventing a cool head and plenty of talent gave defending champion Jonathan Paget, of Auckland, the edge.
Paget and Clifton Promise, who won the title for the second consecutive year, will soon be packing their bags to for America where they will compete in the prestigious Kentucky horse trials next month.
