Well-informed Soviet sources said yesterday they expect Christina Onassis, the Greek shipping heiress, to marry Sergei Danyelovitch Kauzov, a 37-year-old Soviet shipping official, here on August 1.

The sources said the Kauzov family has told them the wedding of the twice-married daughter of the late Aristotle Onassis is being planned for Moscow and that the couple may intend to live here.

Onassis, 27, arrived here June 26 and has repeatedly and emphatically denied persistent reports in the Fleet Street press of London that she intended to marry a Russian soon.

But a report from Athens last night quoted a close family friend. Kostas Gratsos, as saying he had just returned there from Moscow with confirmation that she will proceed with the marriage, which raises complex questions of how she will continue to run the affairs of the vast shipping and real estate empire if she chooses to reside in this isolated, land-locked capital with its notoriously poor communications, and its government which seeks to expand its strategic power at the expense of the West. Her father, who died in March 1975, left her working control of the world-wide family interests, which are estimated to be worth $500 million, and include a large fleet of supertankers crucial to delivering Arab oil to the U.S. and other capitalist powers.

"She is definitely going to marry Kauzov," said a family friend in Athens after conferring with Gratsos, according to United Press International. It is very very sad. It is something her father would have prevented."

Her two other marriages, to a California realtor and to Alexander Andreadis, heir to a rival Greek shipping fortune, ended in divorce or annulment. Her father had tried to prevent the early marriage to the older Californian and was reportedly furious when she persisted against his will.

It was reliably reported here last night that Christina Onassis met Kauzov in Paris in 1976, where he was assigned as an official of Sovfrakht, the Soviet Government organization that acts as shipping agent for the burgeoning Soviet merchant fleet.

Kauzov, who lost an eye in a childhood accident, is said to speak fluent English. Sources said he recently returned from Paris and has just divorced his wife. Natalya. In keeping with Soviet divorce law, she has custody of their child, Katya, 9. The sources said Kauzov had left the family's apartment to his wife - a large consideration of any divorce decree in overcrowded Moscow. He is said to be living with his mother.

These sources confirmed rumors that Kauzov had left his job at Sovfrakht, but suggested it may be only a temporary leave of absence.

Christina Onassis has been seen in Moscow several times by Western correspondents. Her companion each time has been described as a man slightly taller than she, with thinning brown hair. They have spoken English to each other and they have refused to respond to questions from reporters. The two have been seen several times at the self-service food market run specially by the government for foreign diplomats and others, which offers select choices of meat and produce unavailable elsewhere in Moscow and from which most Russians are barred.

The Bizarre notion of a powerful capitalist heiress married to a successful and trusted official of this communist power and possibly living near the Kremlin clearly displeased and alarmed Onassis' Greek countrymen.

"If they were allowed to live abroad, it will be no problem," a wellinformed Greek close to the Onassis family was quoted as saying in Athens yesterday. "But if she has to live there all the time, it will be sad." Greek security officials in particular were said to be alarmed by the confirmation of the impending marriage. Greece was rent by a bitter civil war in which Kremlin-backed communists were defeated in 1949 with massive U.S. aid to royalist forces.

The security officials were said to be gravely concerned over the possibility that the Onassis tanker fleet, one of the world's largest, would fall under Soviet control. "It is one thing for Onassis or Niarchos or any loyal Greek to have their own island," one security official was quoted as saying. "But the idea of a Soviet official controlling Skorpios poses some problems." The Onassis holdings include the island of Skorpios in the Aegean.

Soviet relations with Greece, a staunch NATO member, have been exacerbated since the 1974 Turkish invasion of Cyprus, in which Turkey took control and has partitioned about 40 percent of the island, Turkey recently signed a "political document" of non-aggression with the Kremlin.

Onassis, whose father in 1968 married Jacqueline Kennedy, widow of the slain president, is the only surviving child of Aristotle Onassis, who built his holdings into one of the world's great modern business empires. His other child, a son, Alexander, died in an air accident.

Onassis was originally scheduled to leave Moscow July 2, Greek Embassy officials have said. But she was given an "indefinite extension" of her visitor's visa, according to Greek sources. She visited here once last year, embassy sources said, on what was described as a business trip.

Kauzov, who was said to have acquired a working knowledge of French during his several years in Paris, was said to have been shot in the eye by an arrow at the age of 9 by a playmate. That would cost him his eye.

The Soviet sources were reported to be in contact with the Kauzov family and not to have spoken directly with Christina. Onassis, who has been staying at the Intourist hotel near Red Square during her stay. Her telephone there has gone unanswered for days at a time. One knowledgeable Soviet source said tonight: I'm willing to bet that he does get married."

Under Greek law, the kind of civil marriage encouraged here is not legally recognized in Greece and any progeny from such a marriage are considered to be illegitimate for inheritance purposes. There is one Greek Orthodox and about 50 Russian Orthodox churches in the capital and marriages can be scheduled with relative ease at any of them under circumstances such as these, sources suggested. The Russian Orthodox Church will accept for marriage a prospective spouse who has been divorced up to three times.

Most Soviet marriages are performed at any of three special staterun "wedding palaces," and the brief ceremony is emphatically nonreligious. Newlywed couples are encouraged to lay bouquets at the Red Square mausoleum of Vladimir Lenin, founder of the Soviet state, who died in 1924.

Fleet Street papers have for weeks predicted the marriage of Christina Onassis, but the original report apparently misidentified the prospective bridgegroom, calling him "Nikolai Ivanovitch Kaozov." Onassis in several telephone conversations with reporters has denied any intention of marrying any Soviet, and asserted that she has given no interviews "since my father died."

It has been frequent Soviet practice not to allow Soviet citizens married here to foreign nationals to leave immediately and join their spouses in foreign countries. Emigration is frowned upon an unpatriotic, although numerous foreigners have nevertheless been able to marry Soviets and obtain exit visas for them.

The Soviet merchant fleet has been growing rapidly in the past two decades and Western shipping firms have complained bitterly over the years that the Soviets are deliberately undercutting capitalist fleets to force economic hardship in the West and gain a strategic hold over the flow of goods among Western trading partners. The Soviets have denied this, maintaining that their fleets operate more cheaply because they are more efficient and better run.

A worldwide oil glut caused in part by the sudden raising of crude oil prices by the Arab states several years ago has brought widespread economic stress to the fleets of supertankers. Dozens of them have been laid up and shipyards that once boomed with orders for ever-bigger supertankers have fallen on hard times. The Onassis fleet has not been immune to these pressures.

When Onassis arrived here, Greek sources said it was in pursuit of contracts for several supertankers that were idle.