Once again, the world price of oil is falling -- modestly, compared with the precipitate way in which it went up -- and those with a vested interest in high oil prices are warning that this might lead to an "unstable" world economic situation. Specifically, they say soberly that "too sharp" a decline would make things worse for debtor countries such as Mexico and Nigeria that depend on oil exports.

But don't let crocodile tears over the poor Mexicans and Nigerians fool you: The decline in oil prices will have an enormously beneficial effect on the larger global economy, including such debtor countries as Brazil and the Philippines, which are net oil importers.

The instinctive reaction of the Organization of Petroleum Exporting Countries was predictable: Cut production by 3 million barrels a day, in order "to restore the $29 benchmark price of oil," said Sheik Ahmed Zaki Yamani, oil minister of Saudi Arabia, the biggest and most influential OPEC member. All 13 OPEC country oil ministers will be meeting in Geneva on Monday to parcel out shares in such a cut, assuming that the Saudis will absorb half of it.

Meanwhile, the American government has remained silent on this issue, even though a further price decline clearly would benefit our economy, whereas an artificial effort to maintain prices higher than the market would act as a drag. And some major American oil companies are busy behind the scenes trying to justify OPEC's attempted price-fixing, which provides a convenient umbrella for them.

Coincidentally, Exxon Corp. Chairman Clifton C. Garvin Jr. issued a dire warning about a potentially short supply. "We are not finding as much oil and gas as we consume," Garvin told a Washington Post reporter last week.

Officially, OPEC claims to be producing 17.5 million barrels a day, which is 37 percent of the Free World's consumption of 46 to 47 million barrels a day. That is a spectacular drop from the OPEC peak of 31 million barrels a day in 1979, or 60 percent of Free World consumption of about 52 million barrels a day. But New York oil trader Harry Neustein points out that, after subtracting about 800,000 barrels a day for its own use, OPEC's exports may be no more than 16.7 million barrels a day.

"If OPEC still wants to keep a piece of the market, they should cut the benchmark price by around $5 a barrel," Neustein says.

A little arithmetic shows why: The proposed production cut to protect the $29 benchmark amounts to about 18 percent for OPEC as a whole (much more for the Saudis). The same revenue yield could be obtained by steady production with an 18 percent price cut, which would bring the price down from $29 a barrel to less than $24.

Although many of this country's best-known "experts" refused for a long time to recognize the diminishing economic and political importance of OPEC, it has been downhill for the cartel since 1981, as consuming nations put stunning energy efficiencies into effect, and uncovered new sources.

The North Sea alone now delivers about 3.3 million barrels a day for Norway and the United Kingdom, which exceeds the export level to which the Saudis are now talking about cutting back. There is plenty of other competition for OPEC: The Soviet Union continues to exchange oil for crucially needed hard currency and, to a lesser degree at the moment, so does Peking. Chinese leaded gasoline is turning up on the West Coast at a cost of 69 cents a gallon to refiners, which means that, with an octane boost, it can be sold at the pump for $1.

Yet, there is a reluctance in some political and business circles in this country and in Europe to accept the proven fact that the world's dependence on Middle East oil has fallen dramatically and is likely to continue to do so. In the last decade, the refusal to face the oncoming oil glut led foolish investors into catastrophic investments here and abroad, based on the hope of $90- to $100-a-barrel oil.

Such ill-starred adventures contained the seeds of the Penn Square National Bank disaster, the U.S. government bailout of Continental Illinois Bank, and losses at banks such as First Chicago.

But American policy makers still tread softly: The old image of what was purported to be Saudi power is hard to shake. Imagine the outcry if the world's steel or wheat producers got together and announced that they would try to leapfrog the price mechanism of the market!

The reduced inflationary trend of the past couple of years so eagerly trumpeted as a success for Reaganomics can be traced in large measure to the decline in the price of oil since OPEC's power was broken in 1981. And if the price of oil can be brought down further, encouraging still-lower price indexes, it should bring interest rates down, too. And that will offset at least a large share of the damage lower oil prices will do to oil exporters such as Mexico.

But it is doubtful that the United States will take any positive steps -- and there are some it could take -- that not only would counter OPEC's effort to prop up prices, but also would soften the market even further.

For one thing, the United States could sell oil from its growing Strategic Petroleum Reserve, which now amounts to about 400 million barrels. This not only would push prices down, but also would be a useful exercise to demonstrate how the SPR process would work in an emergency. Such a test never has been undertaken.

For another, the United States could apply a tax on imported oil, which would squeeze consumption further and put pressure on OPEC and other suppliers to lower their prices. To offset any inflationary or regressive result, a compensating reduction in other consumer taxes could be considered.

And the United States should show its displeasure that Mexico and Egypt, two countries heavily dependent on American economic aid or assistance, are flirting with the idea of joining OPEC.

The bottom line is this: By OPEC's own implicit calculation, the real price of oil today should be about $24 or less. That being the case, the United States and other major governments shouldn't allow OPEC to set a price of $29 or any other figure so far above the market level. The Reagan administration shouldn't leave the establishment of such a crucial price to others, especially when they no longer can twist a noose around our necks. It's that simple.