How times have changed for oil, OPEC and the consuming nations. Just two years ago (on March 16, 1980, to be exact) investment banker Peter G. Peterson wrote a sobering piece for The Washington Post Outlook section titled "It's Time to Make a Deal With the OPEC."

Hindsight, to be sure, is a wonderful analytical tool. There were plenty of people other than Peterson who saw nothing but disaster ahead unless the rich and poor nations sat down with the "key player"--as Peterson labeled the cartel--to avoid a growing international crisis, especially for the poor Third World countries that must import oil. Essentially, Peterson's recommendation was an outgrowth of the Brandt Commission exercise.

Peterson's assumption was that the cartel would remain in control of world oil markets, and that the only way to avoid financial chaos was to bow to OPEC's power, and accept more predictable and gradual oil price increases in real terms in return for security of supplies.

The OPEC members of the Brandt Commission were plain in what they were demanding in such a deal. In flexing their muscles, as they played their "trump card"--the phrase was that of Abdullatif Al Hamad, a Kuwaiti member of the Brandt Commission--OPEC expected not only "major flows of resources to the South," but that the industrial nations would pay a tax of $1 per barrel of imported oil for the use of the poor countries.

In other words, at its arrogant peak, OPEC would not reduce the price of oil to help the poor, but would levy a tax to yield $12 billion a year. "That's a lot of money," Al Hamad conceded in a March 1980, interview in an Arab publication. "If the rich countries feel this is too much of a burden, they should consume a little less."

Peterson appeared convinced that OPEC could wield this kind of blackmail. "I was taught at the University of Chicago," Peterson wrote in The Post, "that if one has no alternative, one has no problem. The blunt fact is that over the next five years, we do not have any other alternative for significant ly increased oil supply. Nor, for that matter, have we any other prospect for significant increases in supply from other energy alternatives. Even with vigorous domestic oil exploration development programs, we will be lucky to keep U.S. oil production even at its current level."

Peterson, former secretary of Commerce under Nixon and now chairman of Lehman Bros. Kuhn Loeb, was right on the last point--even though deregulation advocates promised us the moon.

But he was dead wrong on alternatives to OPEC oil and failed totally to visualize the stunning progress in conservation that together have created a tremendous oil glut and have broken the cartel's power to set prices. The new outlook, moreover, promises to reduce, significantly, the financial deficits of the poor Third World countries--that in 1980, when Peterson wrote his piece, threatened to engulf them.

The change since 1980 has been so dramatic that today's essayists are not writing pieces about the dangers of oil prices that are too high, but--ironically--of oil prices that are too low. From a weighted average international price around $28 a barrel when Peterson wrote his Washington Post piece, oil soared in the spot markets to something like $42 at the peak in late 1980, and is now down to $26 or $27, well below the official OPEC price of $34.

Philip K. Verleger Jr., of Booz-Allen & Hamilton Inc., told a blue-ribbon audience assembled here the other evening by the Institute for International Economics that the price is headed below $20. "Cessation of hostilities between Iraq and Iran could create a two- to four-year period of time where the price of oil hovers in the $15 to $20 range," Verleger said.

Thus, what we have witnessed in the past two to three years is a total revolution in the world supply-and-demand equation for oil. A few observers, like Eliyahu Kanovsky and Fred Singer, were prescient enough to forecast the trend, while others were still talking "oil crisis."

The current Morgan Guaranty Bank letter estimates that between 1973 and 1981, oil use per unit of output fell 30 percent in the industrial countries. Four-fifths of this decline--caused by the 500 percent increase in OPEC prices--has happened in the past three years.

"This (downward) adjustment . . . appears (to be) far from complete," the Morgan Guaranty letter states.

On the supply side, non-OPEC production has grown from less than 18 million barrels a day in 1975 to a projected level of 23 million barrels a day this year, more than offsetting a 5 million barrel a day reduction in the combined Iranian-Iraqi production. OPEC, as a whole, has slid from peak sales of over 30 million barrels a day to something like 18 million.

In summary, OPEC is now exporting only 45 percent of the oil consumed by the industrial and Third World Nations, compared with 65 percent in 1974, or "levels below those needed to meet (OPEC's) external financial requirements."

Even assuming a $29-$30 average oil price this year (which seems too high on the basis of current trends), Morgan Guaranty predicts that OPEC, which had an international net cash inflow (current account) of $105 billion in 1980, will slide into a deficit of $10 billion in 1982 and $20 billion in 1983.

But lower oil prices squeezing OPEC will help everyone else, except banks and corporations that believed oil prices would go up, up, up, and never come down, and thus committed resources on the basis of that expectation. Even if prices go no further down, the United States alone would save $15 billion on its oil bill this year, while consumer prices in the industrial nations as a group would be lowered 1 to 2 percentage points.

And as for the Third World countries, on whose behalf Peterson wanted to strike a deal in 1980, lower oil prices will be a boon. Morgan Guaranty estimates a $7 billion saving for 12 major non-oil developing countries this year plus another $5 billion next year. The main beneficiaries, in lower oil prices and better economic growth, will be Brazil, Korea, Taiwan, India and Turkey.

But perhaps the most significant result of the changed oil outlook, as Townsend-Greenspan & Co. Inc. point out in their business-advisory letter, "is its impact on the military importance of the Middle East. The glut doesn't eliminate all dangers of a breakdown in oil availability from the Middle East," the letter notes, but clearly reduces the risk and "the resources" that the United States must devote to it.