Contrary to most expectations, the $29-a-barrel marker price for crude oil set in March by the Organization of Petroleum Exporting Countries appears to be holding, largely because Saudi Arabia has continued to keep down its production and Iran has adhered to the OPEC accord.

But summer is the softest time for oil sales, and some analysts say the market could still easily be upset, even by Iran alone. Iran, which is a bitter political foe of the Saudi kingdom that in the past offered discounts to undermine the Saudis' share of the market and domination of OPEC, has been having trouble selling its oil and may yet be tempted to slash prices, thereby triggering a price war.

U.S. Treasury Secretary Donald T. Regan recently warned that the situation remains "precarious" and said that another drop in world prices could provoke a "second wave" of financial crises for such hard-pressed oil-producing countries as Nigeria, Mexico, Venezuela, Indonesia and possibly even Britain.

OPEC, which groups 13 of the world's larger producers, cut its official price by $5 in March amid much doubting by oil analysts that this would be sufficient to halt the slide threatening many oil producers with financial catastrophe.

Officials of the oil-producing Arab states of the Persian Gulf, however, are cautiously expressing optimism that the price will hold without a mid-year cut that many of them anticipated.

Last Sunday, Kuwaiti Oil Minister Ali Khalifa spoke of a "marked improvement" in the oil market and said "things are now much better than were originally expected."

Similar statements came Tuesday from the United Arab Emirates Oil Minister Mana Said Otaiba and on Wednesday from the Saudi Deputy Oil Minister Fahd Khayal, who announced that the kingdom's oil production was running at more than 4 million barrels a day.

This was the strongest indication that the demand for oil may be picking up, since the Saudis were producing about 3 million barrels a day, or possibly less, two months ago compared to more than 10 million barrels two years ago.

Other signs of improvement include Egypt's May 19 announcement of a 25 cent increase in the price of its two top-grade oils starting Wednesday, an increase in Nigeria's production from 673,000 barrels a day to an estimated 1.3 million barrels in mid-May and an overall increase since February in OPEC production by 1 million barrels to 15.2 million barrels a day, according to the estimates of the respected Petroleum Intelligence Weekly.

Under the March accord, reached after two weeks of bitter wrangling, OPEC members agreed to a production ceiling of 17.5 million barrels of crude and set a quota for each country, with Saudi Arabia called upon to play the role of "swing producer."

This meant that the kingdom would increase or decrease its production by whatever amount necessary to help keep the market tight and thus defend the $29 price per barrel of Saudi light crude. The ceiling set for Saudi production was 5 million barrels a day, still far less than the kingdom needs to meet its already reduced budget.

At the time of the March accord, OPEC production was averaging less than 15 million barrels a day, and there were serious doubts its members would stick to the pricing system as they struggled to find buyers and preserve market shares.

But oil analysts say that they have honored the accord and that Iran, about which there were the most doubts, has respected the $28-a-barrel price set for its crude.

Nonetheless, there have been reports that Iran has been agreeing to marginal discounts in tough negotiations underway with Japan.

The reports, which both Iran and Japan have denied, said that Iranian oil--after being purchased at the official price, was being offered in Japan to refiners at a discount of 20 to 50 cents. Iran was then making up the differences by paying more for Japanese goods purchased through state trading corporations.

Whatever the truth, Japanese firms have agreed to buy less than half of the 350,000 barrels a day they were getting under contracts that expired at the end of March.

Iran's exports have dropped from more than 2 million barrels a day late last year to 1.5 million barrels in April, according to Deputy Oil Minister Abbas Honardoust. Its total production was 1.2 million barrels, compared to its OPEC quota of 2.4 million barrels.

The chief beneficiary in the gulf of the drop in Iranian sales appears to be Saudi Arabia.