An Iranian prisoner of war and a bit of timely "advice" from the Soviet Union have combined to ease the pressure for a large-scale price increase at the OPEC meeting in Bali beginning today, but few experts expect the oil producers to end their meeting without adding to their coffers somewhat.

Oil prices were frozen at OPEC's September meeting in Vienna, except for a $2-a-barrel Saudi increase, but there has been considerable agitation recently by the organization's traditional price leaders for a 15 percent increase. This would set prices in a range of $34.50 to $42.50 a barrel, an enormous jump from the $14 charged just two years ago for top-grade crude.

At the optimistic end of the scale is a 5 percent increase, or about $2 a barrel, with Saudi Arabia again exerting pressure on other members of the Organization of Petroleum Exporting Countries to hold the line.

None of this approaches the doubling of oil prices during 1979 that sent shock waves through the world economy, and none is certain given the unusual situation of an OPEC meeting taking place while two of its most prominent members are at war. The conflict between Iran and Iraq has brought about as much uncertainty for the oil producers as it has for the consumers.

The war, approaching its third month, has cut the amount of OPEC oil that experts expected would be available by about 2 million barrels a day, forcing the major industrialized countries to use more of their reserves than originally planned to avoid a frantic scramble on the open market that inevitably would lead to a sharp price increase by OPEC.

But just as the war created the highly unusual conditions in the world oil market, it apparently has provided some relief in the form of tacit understandings that allow each country to resume some oil exports, according to well-informed sources.

The capture of Iranian Oil Minister Mohammed Javad Tondguyan while on a tour of the front has given Iraq leverage it needed to resume pumping oil through the pipeline from its Kirkuk fields to Turkey and the Mediterranean. The Iranians previously had demonstrated the ability to knock out the pipeline. The Iraqis reportedly have let the Iranians know that Tondguyan essentially is being held hostage against the smooth operation of the pipeline.

Similarly, the West has received help from Moscow in the form of pressure on Syria to reopen another pipeline that passes through its territory from Iraq.

The Syrians, bitter enemies of Iraq, have just signed a treaty linking them more closely to Moscow. According to one account, the Soviets suggested the pipeline be reopened in part because Eastern European countries were suffering under the cutoff of Iraqi oil, and the Soviets were in no position to make up the difference without serious impact at home.

Using both pipelines, Iraq is now said to be exporting about 700,000 barrels of oil daily with a potential of boosting that figure to 1 million barrels. Iran is reported to be sending out about 300,000 barrels daily, some of it via Kharg Island in the war zone, a sign that the generals who were pounding each other's oil facilities may have been curbed somewhat.

This is still far below the 3.8 million barrels daily that the two were exporting before the outbreak of hostilities. An extra 1 million barrels in daily production by other OPEC members still leaves the world with about 2 million fewer barrels daily.

Those 2 million barrels were being put into storage, providing the oil "glut" that militated against sharp price increases earlier this year, but that was a cushion based on projections for the third quarter of the year, traditionally a period of relatively light use of oil.

It was a cushion that experts had predicted would drop to 1 million barrels a day during the current quarter and disappear in the first part of 1981. These were projections made before the Iranian-Iraqi war, according to experts in the field.

When the war broke out, cutting off all exports from the two countries, the glut of oil turned into what one expert called a "glut of uncertainty" that made many companies, and some countries, highly nervous about dipping into their huge stores. The result was pressure on the open, or "spot," market that put new pressure on prices generally.

With an eye to forthcoming OPEC meetings, officials in the major industrialized countries, under the umbrella of the International Energy Agency, have labored to maintain an air of calm and pledged to cut consumption by 10 percent during the first three months of 1981 and to rely more heavily on their stockpiles.

This is a pledge that carries more weight with the resumption of exports by the warring countries at 1 million barrels a day, combined with increased production from non-OPEC countries and a more rapid decline in demand in the West as Europe enters an economic downturn deeper than expected by many forecasters less than a year ago.

All experts quickly caution that normal logic seldom applies in a wartime situation, especially one involving an organization full of conflicting pressures such as OPEC.

Libya, Nigeria and Algeria -- price "hawks" within the organization -- are expected to push for maximum increases at the Bali meeting. Iraq, which had been lining up with Saudi Arabia recently on the side of moderation, signaled Friday that it might be willing to go for increases of about of 10 percent, roughly in the middle of projections made so far.

Iraqi Oil Minister Tayeh Abdul Karim said in Kuala Lumpur, Malaysia, during a stopover on the way to Bali, that he supported the Indonesian call for a 10 percent increase and that Iraq was "in the majority" on the price issue.

As the oil ministers gathered yesterday in Bali, Venequela's Humberto Calderon Berti, generally among the moderates, aired a proposal for a 5 percent increase on Jan. 1 to be followed by other increases during the course of the year that would total 17 percent.

All this presumes that the meeting will not break down into a shouting match between the Iranians and the Iraqis, an assumption upon which no one was willing to place heavy bets.