THE DROP in oil prices is the industrial world's reward for energy conservation. The world is using less oil than the producers had expected, and all those agreements and understandings about price maintenance are beginning to come unraveled. That's unambiguously good.

In the 11 years since the first great rise in oil prices, people in this country and most others have gone to great lengths to do more with less fuel. Here in the United States consumption of oil and natural gas is down by one-third, in relation to GNP, since 1973. Most of that gain has been achieved since the second big rise in prices following the Iranian revolution five years ago. Cars go farther on a gallon of gasoline. Industrial machinery provides more for each kilowatt of power. People are more careful with their thermostats. It's paying off.

Early summer was traditionally the time for the oil-consuming countries of the northern hemisphere to begin building up their stocks for the winter. It hasn't happened this year. That has meant fewer customers for the producers. The political pressure on the producing countries to drop prices has been most visibly severe in Europe.

Last July the British National Oil Company began having trouble selling its North Sea oil at its price of $30 a barrel, but when it suggested a price cut the British government vetoed the idea. The government was protecting its revenues, at the expense of British industry and consumers -- a decision that drew a good deal of hostile comment. A few days ago, the Norwegian state oil company cut its price to $28.50. This time the British immediately dropped into the same range. Next Nigeria, whose oil competes directly in quality with the North Sea's, cut further to $28. Now OPEC has anxiously called an emergency meeting.

OPEC is in a bad position. Its excessive price increases of the past decade have brought into the market major new producers, most conspicuously Britain and Mexico, that remain outside the cartel. If OPEC wants to maintain its present prices, it will have to cut its own production again -- and most of the Arab countries already have their oil exports down around half the level of 1979, the last big year. Nigeria evidently does not intend to reduce production further. That confronts OPEC's Arab members with an unpleasant question: are they prepared once again to absorb all the cuts in produc maintain the present world prices?

The past decade has seen a gigantic transfer of wealth from the consumers of oil throughout the world to the producers. With this decline in prices, the transfer will slow down slightly. That's reason for a certain modest amount of celebration -- and for further conservation.