A SECOND WAVE of strikes in Iran raises once again that unpleasant question: What if the world has to get along with a suddenly diminished supply of oil? The first strikes, a month ago, had hardly ended before the present round began. Iranian oil production is down to about half the normal volume and, apparently, sinking fast. Until recently, the world was pumping and burning a little over 60 million barrels of oil a day. Nearly one-tenth of it came from Iran's wells.While losing an amount of that magnitude might not be catastrophic, it would be decidedly uncomfortable. Worse, it would increase the instability of the world oil system by increasing the industrial countries' dependence on the other great exporter, Saudi Arabia -- a dependence already dangerously great.

The United States does not import huge amounts of Iranian oil. Iran usually ranks fourth or fifth among this country's sources of crude oil, after Saudi Arabia, Nigeria and, depending on the shifting patterns of the trade, Libya, Algeria or Indonesia. But Iran is directly and indispensably important to Japan and Western Europe. If Iran's deliveries fall off, or continue to be irregular because of the country's internal chaos, the shortfall in supply will have to be shared among all buyers in both hemispheres.

During the first oil-field strikes in November, Saudi Arabia quietly raised its production to cover a large part of the drop in Iran. The Saudis are evidently prepared to prevent the disruptions in Iran from inflicting a severe shortage on the world, with all of its implications for jobs, economic growth and financial stability. But it is necessary to expect that supplies may be a little tighter this winter than the importing countries had expected. Earlier this year there was talk of a worldwide oil glut -- meaning that the exporters had put a little more crude oil on the market than their customers immediately needed. But it was a very insubstantial and evanescent kind of a glut. It was always very small compared with the amounts of oil that the world is using, and it always depended on a political decision by one government -- Saudi Arabia's.

The Saudis are the only nation with both the physical capacity and the political latitude to raise or lower their production by significant volumes. That's why they now have more influence over oil prices than anyone else. OPEC, incidentally, is about to meet and raise the price again. The Iranian strikes are already beginning to push certain prices upward, and it will be difficult for OPEC to resist the temptation to exploit its opportunity.

Americans sometimes speculate, hopefully, that the market will shortly be awash in oil from those countries with large reserves but so far low production. Iraq and Mexico are the examples most frequently cited. It's a nice thought, but improbable. Those countries are now explicitly restricting their oil sales to the amounts they need to finance their own carefully measured development plans. Mexico, in particular, may well choose to sell more of its oil abroad in the years to come. But it is unlikely to move either fast enough or heavily enough to offset the effects of any prolonged interruption of the Iranian supply. Iran, after all, has become notorious as an example of spendthrift national policy. Oil-producing countries, and their citizens, have become incareful use of resources. They were an element in this week's election in Venezuela, for example, where the voters turned out the government on the charge, among others, that it had been wasting the country's oil revenues.

The declining flow of oil from Iran this week invites, inevitably, a glance backward to 1973 and the Arab oil embargo. Alone among the world's major industrial nations, the United States has allowed its oil imports to rise over the past five years; they are one third higher now than then. But, as events are showing, wars and deliberate embargoes are not the only threats to a fragile and overburdened line of supply from the Persian Gulf.