STRIKES AND POLITICAL demonstrations in Iran have now cut off most of its tremendous flow of oil exports. That constitutes another warning to the United States of its dependence on an oil delivery system vulnerable to many kinds of disruption. Iran is the world's second-largest exporter of oil, ranking behind only Saudi Arabia. Its production has been in the range of 5.5 million barrels a day - one sixth of OPEC's total production, one eighth of all production in the world outside the Communist countries.

Through a piece of simple luck, the interruption of Iranian oil will not have an immediate effect on the rest of the world. Because OPEC plans to raise its prices at the end of the year, oil companies have been buying as much at this year's prices as they can get. Inventories are currently high, and pipelines are full. But if Iran's great oil terminals were to remain shut down for more than a few weeks, the effects might soon mean shortages.

How serious would the shortages be? It's impossible to tell, because that would depend very largely on the response of other members of OPEC - the Organization of Petroleum Exporting Countries. Specifically, it would depend on Saudi Arabia. Because high prices and slow economic growth have restrained the demand for oil around the world, the Saudis have cut back production. They are the balance wheel of the OPEC cartel, taking on themselves most of the responsibility to reduce oil exports and keep prices up. Current Saudi production is far below capacity - by most reckonings, at least 3 million barrels a day. Other Middle Eastern exporters also have an ability to raise shipments, if they choose.

Past Saudi policy suggests that they would probably put enough additional oil onto the market to prevent any real crisis - but not enough to avoid a certain stringency. In recent months, the market has been slack. There has been a bit more oil offered for, sale than the buyers need. The Saudis aren't likely to perpetuate that comfortable condition.

The Iranian disruptions ought to remind Americans of the value of a Strategic Petroleum Reserve - if only they had one, in more than name. A long shutdown of the Iranian oil industry is precisely the kind of emergency for which the strategic reserve was designed. Unfortunately, the rate of filling the reserves - in underground caverns along the coast of Louisiana - has fallen far behind schedule, and there's hardly enough in storage currently to provide any significant insurance. Every country that uses oil now has the most direct kind of interest in the reestablishment of political stability in Iran. But if the turmoil there continues, the United States - having been slow to protect itself with reserves - must rely once again on the good will of the Saudis to forestall the possibility of serious and damaging oil shortages.