While declining world oil prices have had a direct and adverse effect on the Soviet Union, the world's largest oil producer, they have unexpectedly opened possibilities for Moscow for potential political gains in the Middle East.

Facing a drop in its hard currency earnings, Moscow has been scrambling to increase its share of western oil markets with increased supplies and lower prices.

The Organization of Petroleum Exporting Countries, watching a growing number of Soviet tankers heading for Rotterdam and other western refining centers, is now reported to have approached Moscow in an effort to establish "links" that would stabilize the world oil market.

Diplomatic observers here believe that the Soviets are going to exploit the opportunity to reach out toward moderate and conservative Arab states through private channels. The OPEC approach, one diplomat said, "now gives Moscow a chance to show its importance to the Arabs not only in the political but also in economic context."

A spokesman for OPEC, United Arab Emirates Oil Minister Mana Said Otaiba, was quoted Sunday as saying that the OPEC coordinating committee has asked Algeria, a member of the 13-nation cartel, to approach Moscow in an effort to create "links" for possible coordination.

Oil minister Humberto Calderon Berti of Venezuela, another OPEC member, was quoted earlier as saying that the cartel should start talks with Moscow "in the near future." Venezuelan Foreign Minister Jose Alberto Sambrano arrived here Sunday on an official visit during which he was expected to discuss the matter with his hosts.

The Soviets have been selling their crude on the Rotterdam spot market for between $27 and $28 a barrel. They have also undercut competitors by reducing prices they charge to regular western customers.

Over the weekend, Moscow was quoted by wire services as having proposed a 50-cent increase in its price, a gesture toward stabilizing the market while still selling below the OPEC price.

In an effort to halt a deteriorating market, OPEC last month agreed to peg the total output of the cartel to 17.5 million barrels a day and to cut its benchmark price from $34 to $29 per barrel.

Diplomatic sources here expect the Soviets to enter into talks with OPEC and to privately press for contacts with conservative Arabs states, particularly with Saudi Arabia, the world's largest exporter of oil. Moscow has repeatedly hinted it wants to establish ties with Saudi Arabia, something the staunchly conservative kingdom has refused to do thus far.

There has been no official information here about Soviet oil policies except for an article by a senior Soviet economic expert warning Moscow's Communist allies that they should not expect any increases in the amount of oil they import from the Soviet Union.

"We have clearly reached the limit in output of oil and a number of other components of our energy and raw material exports," Oleg Bogomolov wrote in the monthly International Life. He added that these limits did not apply to natural gas and electrical energy.

Bogomolov said Moscow's allies must improve the way existing oil supplies are used and rely on domestic resources and nuclear energy for additional requirements.

The Soviet Union's oil production last year was 613 million tons, or slightly over 12.1 million barrels a day. It marked slightly less than a 1 percent increase over the 1981 oil output. In 1981, the Soviets exported 3.3 million barrels a day, but well over half of this amount went to their soft currency partners.

Soviet oil exports to Western Europe increased slightly in 1982, going primarily to Finland, France, West Germany, Italy and the Netherlands. This year, Soviet sales in the spot markets have been especially intensive.

The importance of oil exports for Moscow's hard currency earnings can be measured by statistics showing that crude oil accounted for more than half of its trade earnings in 1981 and, according to western estimates, may have been close to 60 percent in the last year.

Working in a declining market, the Soviets are believed to be aiming to shift a large proportion of their total oil exports into western markets. This could be accomplished by renewed conservation efforts at home and greater restriction on exports to soft currency markets. The hard currency revenue is used to pay for Soviet imports, including large quantities of grain as well as equipment and steel pipes used in the construction of the 2,800-mile Siberian natural gas pipeline.

Western experts here say that the increased Soviet oil sales are expected to offset the impact of falling prices. Moreover, Moscow's balance of payments picture is healthy since the Soviets have been following a generally prudent policy following the nearly 10-fold increase in oil prices in the 1970s, avoiding large hard currency debts.

It is believed here that Moscow's interest in stabilizing the oil market is parallel to that of OPEC. But the Soviets are expected to seek political advantages in dealing with OPEC whose members may be vulnerable because of having run up massive debts in recent years on the basis of projected oil revenues.

Moscow, in particular, would like to establish direct contacts with the Saudis. Saudi Arabia has spurned repeated Soviet overtures, including the late Leonid Brezhnev's warm message to King Fahd last fall and Yuri Andropov's equally strong bid for better Soviet-Arab relations during a visit here last December of an Arab delegation that included Prince Saud, the Saudi foreign minister.

The prince was the first Saudi official to visit here since his father, prince and later king Faisal, paid a visit to Moscow in 1932. The Soviet leaders promised Saud to send him a copy of a 1932 Soviet film about his father's visit. It is believed that the film had already been sent to the Saudi foreign minister.