WOULD IT BE churlish and ungrateful to consider, for a moment, why OPEC did not choose to celebrate the New Year with another price increase? After all, the cartel has done just what the United States wanted it to do. The expressions of official satisfaction here are heartfelt. The present price is certainly preferable to a higher one. Perhaps OPEC's decision is a pure gift, in the spirit of the season - Best Holiday Wishes From Your Friends in the Persian Gulf. Perhaps its restraint only reflects the slight surplus of oil that has made world markets a bit slack lately. But oil surpluses, like best wishes, come and go quickly in international politics. There are other aspects of this week's OPEC meeting that ought to make Americans decidedly uneasy.

The Shah of Iran, for example, has reversed his usual position. Last year he wanted a big increase while the Saudis held out for a small one. OPEC split, and oil was actually selling at two prices throughout the first half of this year until a compromise was patched together. But this year both Iran and the Saudis declared themselves in favor of no increase at all. Between them they account for half of OPEC's production, and when they agree, the smaller producers have no choice but to go along. The Shah had a reason, of course, for changing his mind.

The Shah's appetite for American military equipment, particularly aircraft, is inordinate and continuous.Earlier this year he had great trouble clearing the sale of the huge airborne command stations known as AWACS. Congress eventually decided not to veto the sale, but the controversy demonstrated that there was an uncomfortable amount of opposition to it. Now the Shah wants large numbers of the most advanced American light fighter-bomber. When he was here in Washington last month he said breezily that there is no connection at all between arms and oil. But that is only the sort of absurdity that statesmen sometimes utter to remind their audiences that the opposite is true.

The Saudis, meanwhile, what a fleet of the latest American interceptor plane. THe enormous accumulation of very sophisticated American weapons in the Middle East raises questions to which nobody in the United States has any useful answer. What if someone should begin to use these elegant machines for the purpose for which they were designed? Present U.S. policy is limited to hoping very hard that nobody will do that.

The Saudis' insistence on price stability has not been popular within OPEC. But they have offset that irritation with an extremely valuable promise to their partners in the cartel: The Saudis have committed themselves not to raise their production next year above 8.5 million barrels a day. That's substantial drop from this year's level, which has been around 9.5 million barrels a day. The Saudis have promised, in effect, to take the surplus off the market and, if world demand picks up a little next year, to let their smaller partners have the full benefit of it. By holding firm on prices but cutting production, the Saudis have managed simultaneously to ingratiate themselves further with the United States and to strengthen OPEC for the future.

OPEC sees the next couple of years as a difficult time. Production will be rising rapidly in Alaska and the North Sea, holding down demand for Persian Gulf oil. But worldwide consumption of oil is rising steadily, largely because the United States keeps using more of it. Most forecasts indicate that this rising consumption will suddenly stretch markets very tight sometime between 1980 and 1982. Then the world's buyers will have nowhere to turn but the Persian Gulf - and, as the bidding for Persian Gulf oil gets increasingly desperate, the United States will have less and less discretion over arms sales. The oil exporters understand these factors perfectly, and are playing a patient game. President Carter also understands the forecasts - but has not yet found a way to expplain them to the rest of his country.