The married dad-of-three, who died yesterday aged 73 in France, began a four-year affair with the princess during her failing marriage to his friend Prince Charles after her dad died in 1992.
Diana is believed to have been besotted with the dashing art dealer, who "consumed her every thought" and "got her into sex".
She was so infatuated she smuggled her lover into Kensington Palace in the boot of her car.
Oliver was once found semi-naked behind a potted bay tree in a corridor clutching a cigar while the fire alarms sounded, according to Diana's former royal protection officer Ken Wharfe.
But when Oliver, an Old Etonian who was 16 years her senior, refused to leave his wife and ended the affair, Diana apparently became the ultimate woman scorned, reports The Mirror.
A close friend of Diana told the publication how the "naughty" princess would drive to his house, park up outside and call his landline from her mobile.
“She’d do it in the middle of the night, so she could watch the lights in the house go on as he scrambled to the phone," the friend recalled.
"She would be there for hours, watching and calling. She knew she couldn’t be with Hoare, because he was married.
"And while he might have been a gentleman, and a very entertaining one at that, it was his wife who actually had the money."
The friend admitted that Diana knew Oliver would never leave his wife, but she wasn't used to not getting her way and would enjoy "taunting" him to remind him she was there.
Diana admitted in the Panorama interview with Martin Bashir in 1995 that she had sometimes called Oliver.
“Over a period of six to nine months,” she said. “A few times, but certainly not in an obsessive manner, no.”
The friend claimed Diana enjoyed playing games and would even ask her staff to ring Oliver's home number on her behalf, telling them to hang up.
She added: "He was playing with fire when he started dating a princess, and he should have known he could get burnt."
Oliver never spoke about their relationship in public, and has taken the full story of their passionate affair to his grave.
A police officer at Kensington Palace revealed Diana would go to great lengths to see her debonaire lover, and her increasingly erratic behaviour raised eyebrows among royal staff and security.
Diana’s friend Lady Elsa Bowker once claimed she “daydreamed of living in Italy with the handsome Hoare”.
Another friend allegedly said palace staff knew when Oliver was coming because Diana ordered in Krug champagne, despite the fact she didn't drink.
"They would love a quiet dinner together," the friend said. "They were quite boring when they got together, but Diana liked that."
To keep their affair under wraps, Oliver urged Diana to get rid of her royal protection officers, however by the time she did it was too late.
Both Prince Charles and Oliver's wife, French oil heiress Diane de Waldner, had learned of the affair, which proved fatal for their liaisons as Oliver was desperate to make his marriage work.
Some claimed at the time that Charles welcomed the relationship for taking the heat off his own dalliances with Camilla Parker Bowles, which had been exposed in a bombshell book by Andrew Morton in May 1992.
Charles and Diana separated officially in December 1992, after years of speculation that they were sleeping in separate rooms and holidaying apart. They divorced in 1996.
Diana had already enjoyed several flings including with her bodyguard Barry Mannakee and a five-year affair with cavalry officer James Hewitt.
Oliver’s wife threatened divorce but agreed to give the marriage another try — if he ended his affair with Diana.
We previously told how psychic twins claimed Princess Diana has spoken from beyond the grave to predict Meghan and Harry will be "blessed with a child before long".
And never-before seen letters reveal what Prince Philip really thought about his former daughter-in-law.
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