Cassini died on Long Island, said Senada Ivackovic, marketing director for Oleg Cassini Inc. The cause of death was not immediately known.
Kennedy, only 31 when her husband was elected president, was the pinnacle of style in the White House years from 1961 to 1963. Her simple, geometric dresses in sumptuous fabrics, her pillbox hats and her elegant coiffure were copied by women from 18 to 80.
Cassini said that shortly after John Kennedy was elected, he persuaded his wife that she should use him as the creator of her total look, rather than one of many designers. The one-time Hollywood costume designer turned couturier had been friendly with the Kennedy family for years.
"We are on the threshold of a new American elegance thanks to Mrs. Kennedy's beauty, naturalness, understatement, exposure and symbolism," Cassini said when his selection was announced.
The fashion establishment was shocked, Women's Wear Daily journalist John Fairchild wrote in his 1965 book "The Fashionable Savages."
"Everyone was surprised," he wrote. "Oleg Cassini had been around for years. He was debonair, amusing, social, but none of the fashion intellectuals had considered him an important designer."
Cassini was born in 1913 in Paris to wealthy, aristocratic Russian parents who were later forced to flee their homeland after the Revolution. They settled in Italy, their fortune gone, but his mother gained some success as a dressmaker and her son eventually decided to go into the fashion business, too.
Cassini came to the United States in 1936 and held various design jobs in New York before going to Hollywood and landing a job at Paramount in the early 1940s.
With the fame that came with his White House assignment came new business opportunities. He was one of the first designers to pursue licensing agreements that put his name on a large variety of products from luggage to nail polish.
Although the first lady sometimes wore clothes by others, Cassini provided the bulk of her wardrobe, later saying he had created 300 outfits in the less than three years of the Kennedy administration.
"In Hollywood, I was used to getting a script and a star and they'd say, 'Do it,"' Cassini told The Detroit News in 1995. "Now, with her, it was the same thing. I had to create a persona."
The strategy created a sensation from the beginning. Jacqueline Kennedy's Inauguration Day outfit of a fawn-colored wool coat with a sable collar, over a matching wool dress and a pillbox hat, launched millions of copycat outfits.
"The other ladies wore fur coats, and they looked like bears," Cassini recalled years later.
Overseas trips were a particular challenge, with wardrobes carefully designed to echo the local culture.
"The planning was constant, the logistical invasion of every country she visited, every party she attended — the cloth, the weather, the sensitivity of the people and what they wanted to see her in," Cassini said in 1995.
In his 1995 book, "A Thousand Days of Magic, Dressing Jacqueline Kennedy for the White House," Cassini recalled a constant sense of urgency during the White House years.
"All I remember about those days are nerves, and Jackie on the phone: 'Hurry, hurry, Oleg, I've got nothing to wear,"' he wrote.
In the years following Kennedy's assassination, he saw Jacqueline Kennedy only sporadically. When she died in 1994, Cassini called her "a woman of extremely good taste, a marvelous influence in the arts, in furniture, in food and in clothes. ... She created fashion because she was who she was."
In the 1990s, Cassini launched a partnership with David's Bridal.
His dresses were displayed at the Metropolitan Museum of Art in 2001 in its exhibit "Jacqueline Kennedy: The White House Years."
Cassini reflected on his life in a 2005 interview with The Associated Press.
"I'm doing things the way I've been doing them," he said. "Most men that I compete against put a stop to their career when they become typical."