Born in the US, he has lived in Britain for 25 years, but says he cannot get used to British deference to royalty. "Fergie is just a dim girl who happened to marry someone in the Royal Family," he says. And of the High Court judge who originally put a temporary injunction on the book, he recalls: "He was speaking in hushed tones, saying he was very worried what the Queen might think of all this. I sat there thinking, `Blimey! Where are we here?' "
O'Mara is an old hand at ruffling the Royals. He was the man behind Andrew Morton's Diana: Her True Story which in 1992 blew the whistle on Charles and Diana's marriage, hastening the announcement that they were to separate.
The book was the stuff of publishers' dreams. In its first year, 4.5 million copies were sold world-wide, with translations in 23 languages. In O'Mara's office in Clapham, south London, the shelves are lined with different editions. Morton and O'Mara made millions from it.
O'Mara, 52, says the secret of his success has been to do royal biographies seriously, with rigorous research and no idle speculation or tittle-tattle. "I think I've tried to set a new standard in royal publishing," he says. "Royal biographies have to be more like political biographies when you are trying to give an honest picture of someone's political career.
"I get approached by dozens of people who have the inside story on something or other. But generally speaking they are of the Madame Vasso type and I don't think it's proper to publish that kind of material."
The Diana book radically changed the style and content of royal bestsellers. O'Mara thinks readers are no longer satisfied by books based on what "a Palace source says" or "I understand from a close friend".
The book also had a profound effect on the way the tabloid press reported the Royals. "It totally altered the landscape," says Andrew Morton. "The days when a tabloid editor would say to his royal reporter `I want a royal splash for Monday and I don't care if it's true' are gone. The Sun was commended recently for the accuracy of its royal stories. In a curious way the whole mythology of monarchy has broken down and people do want the truth."
The idea that O'Mara has set a new standard in books about the Royals might astonish the Queen. But there was a time when he was the Palace's favourite publisher. When he set up Michael O'Mara Books in 1985, his first project was a tie-in with ITN celebrating the Queen Mother's 85th birthday. The book was an instant best-seller. Later, Prince Charles asked him to do another, based on a TV interview with him and Diana. Again it topped the best-seller list for weeks.
Glossy photo books on Diana became O'Mara's bread and butter. "I did books on how gorgeous she is, what pretty frocks she wears, hasn't she got a nice hairdo. That kind of thing." He even did a complimentary one on Fergie called Duchess. "Good old-fashioned royal books," he calls them now. But Morton's Diana killed off the lucrative market in deferential royal glossies. "I guess I kind of spiked my own pitch," he laughs."Those books are dead. The industry has disappeared."
Royal publishing has instead become a cut-throat industry. Before Morton's book came out, the world's press were desperate to get their hands on it. O'Mara's office was broken into and journalists camped outside his Dulwich home. Fearful of leaks, he had the book printed in a remote part of Finland. The size of his operation helps with secrecy. There are only 20 full- time staff, including his wife, Lesley, who is head of sales. The Fergie book, now due out on 4 November, is still on disk, being edited in a secret location.
But books on royals are no longer his mainstay. He is publishing about 70 titles this year and the Starkie book is the only one in that category. While his scoops may be the envy of the publishing world, he says he's not interested in doing more royal splashes.
Even though he has emerged the victor in the battle to get Dr Starkie's book published, it has left him cynical about British law: "I'm a pretty strong freedom-of-speech man. I think the power of injunction which exists in this country is an outrage. And there's no chance of getting proper freedom-of-speech laws here because people are lined up in a row to prevent it: MPs, the judiciary, the Royal Family, the establishment in general. It's there to protect the elite, the super-wealthy.
"You know Fergie has had four other, successful injunctions out this year. She is becoming the Robert Maxwell of the Nineties. But she was fighting people who don't happen to have a few hundred thousand pounds. I do, and that's the difference."
Andrew Morton thinks O'Mara is one of a "long line of Americans, from Wallis Simpson to John Bryan, who's helped to inadvertently or otherwise bring down the House of Windsor".
But O'Mara scoffs at the idea that he is "the man who blew the lid off the monarchy". "For heaven's sake!" he says. "If ever there was a case of self-demolition, it's the House of Windsor. They are such an anachronism, 100 years past their sell-by date."