IN MY OWN FASHION: An Autobiography By Oleg Cassini Simon and Schuster. 370 pp. $19.95

AS OLEG CASSINI writes in this self-congratulatory but generally accurate autobiography, "I have dressed (and undressed) some of the most significant women of the twentieth century. I have been of service to American royalty (the Kennedys); I have done well in business and I have had a life filled with romance, excitement and glamor."

Cassini was born in the terminal years of Russia's czarist regime. One of his earliest memories is the shooting of his cousin by mutinous sailors "just as we were about to go down to tea."

As the first born scion of the Capizzucchi-Cassinis, "descended from Crusaders on the one side and Teutonic knights on the other," he was enrolled at birth in the Imperial Guards, whose members were expected to be "master horsemen, swordsmen, chess-players and, also, I assume, passionate lovers, linguists, dancers, gourmets, etc."

His father, Count Alexander Loiewski, was an eccentric figure, who "spent much of his time eating or preparing to be fed" and being fitted for the clothes that occupied a major part of his life; he was the owner of several hundred shirts, all silk and of varied colors, that he would send to London, 50 at a time, to be laundered. When he left for Siberia to join Admiral Kolchak and his White forces in their vain resistance to the Bolsheviks, he had a pair of high boots specially made, "knee-length and lined with tiger skin."

During the count's prolonged absence the family was held together by the Countess Cassini, a formidable survivor who (as the debutante daughter of the Russian ambassador) had caused something of a flurry in Washington, D.C., during the McKinley and Roosevelt administrations. On proceeds from the sale of the few remaining family heirlooms and "a loan from Queen Sophia of Greece," she opened a succession of boutiques, first in Switzerland and then in Italy, that allowed her to give her two sons a good European education (complete with private tutors). At no time were they allowed to forget that they were "gentlemen of noble lineage, assured of acceptance among their peers in the meeting places of the elite."

For Oleg Cassini, this knowledge proved of the utmost importance, particularly when added to the four inherited characteristics that have played such a motivating part in his life:

1) Fascination with clothes (male and female) combined with a talent for design that gave him a fall-back in some of the most precarious times of his life and contributed to his final triumph.

2) Athletic prowess, notably in horsemanship and tennis, that stood him in good stead throughout his variegated career.

3) A passion for combat (first revealed when he thrashed a school bully in Florence) which he continued to induldge for the rest of his days. Equating his nightclub brawls at Ciro's, El Morocco and the Stork Club with the exploits of his noble ancestors, he assures us that "fighting was bred in my bones. I came from a warrior class: My ancestors on both sides have been warriors from the steppes of Asia, Mongol horsemen, Cossacks and Italian Crusaders. My ancestors excelled in combat, and the urge to fight, I believe, was imprinted in my genetic material.

4) Last but not least -- a lifelong preoccupation with the fine art of seduction in which he does not hesitate to rate himself with such accredited masters as Casanova, Don Juan, John Barrymore, Errol Flynn and Rubirosa. After 50 years he remembers with pride and satisfication "the strategies -- the infinite subtleties and challenges" of some of his amorous campaigns.

IF I HAVE felt it necessary to dwell at some length on Cassini's inherited characteristics, it is because many of the incidents recorded in this picaresque autobiography would be difficult to understand or believe without some awareness of the mythology that underlies them.

Cassini's apotheosis and the fulfillment of his wildest hopes occurred in the early '60s when Jacqueline Kennedy chose him as her exclusive designer. This gave him access to the White House ("through the front door and not through the servants' entrance") where, in addition to his esthetic responsibilities to the first lady, he frequently found himself called upon to entertain the president after hours. With the same meticulous care with which he planned his seductions he designed conversations that he sensed the president would enjoy:

"I could draw on the experience of a lifetime and discuss the Russian mind, the Kama Sutra, American Indians, the difference in the appeal of Tyrone Power and Errol Flynn or the relative merits of Marilyn Monroe and Jayne Mansfield."

Cassini's life-story swings wildly between disaster and victory, courage and folly, humiliation and triumph, all spiced with innumerable "names" and reported in a style that is a curious mixture of sincerity and braggadocio, indiscretion and decorum. Although it contains at least one fight and one seduction in every chapter, the strictest censor could not give In My Own Fashion a rating other than P.G. (Parental Guidance Suggested.)

John Houseman is a producer, director and Academy Award-winning actor. His most recent book is "Entertainers and the Entertained."