VIENNA, JUNE 26 -- OPEC ministers today agreed to hold the benchmark oil price at $18 a barrel through the end of the year.

Rilwanu Lukman, president of the Organization of Petroleum Exporting Countries, and Indonesian Oil Minister Subroto said final production figures for the fourth quarter of this year would be between 16.6 million and 18.3 million barrels a day. Lukman confirmed that the actual fourth-quarter pumping figure had not yet been decided in OPEC talks here.

OPEC had provisional plans to boost production from the current 15.8 million barrels a day to 16.6 million barrels a day in the third quarter and to 18.3 million barrels a day in the fourth quarter.

Many OPEC experts and some oil ministers contend that the proposed fourth-quarter ceiling is too high to maintain the cartel's $18-a-barrel price. "There is indeed the possibility to have a {production figure} below 18.3 {million barrels a day} in the fourth quarter to support prices," Subroto said. "Our feeling is this is something that will get support. Everybody wants to retain the $18 price."

Subroto said there was no strong opposition to raising the ceiling to 16.6 million barrels a day in the third quarter.

OPEC price hawks Iran and Libya adamantly opposed the proposed increase in the fourth quarter because Iraq is overproducing and threatens to undermine prices.

"There is a common position with almost the majority of member countries," Iranian Oil Minister Gholamreza Aghazadeh said today. ". . . I can say there will not be such a figure as 18.3 million barrels a day for the fourth quarter."

Iraq, which has been at war with Iran for seven years, refused to sign an OPEC accord in December that limited production to 15.8 million barrels a day for the first half of 1987 and boosted oil prices by $3 to an official average of $18 a barrel.

Cartel nations are producing about 17 million barrels a day -- well above its self-imposed cap -- primarly because Iraq is exceeding its OPEC-assigned quota of 1.46 million barrels a day. Iraq is pumping about 2 million barrels and plans to increase that by another 500,000 barrels a day later this summer.

A Middle East oil source said that if OPEC adopts the 18.3 million barrel-a-day lid for the fourth quarter, production could rise to more than 20 million barrels a day because of cheating.

Middle East oil sources said Saudi Arabia and Kuwait have threatened to cut off the 300,000 barrels a day of war-relief crude that they sell for Iraq if Baghdad proceeds with plans to pump an additional 500,000 barrels a day through a new Turkish pipeline.

Saudi Arabia wants the price to remain at $18 a barrel until the end of the year to spur the modest rise in world oil demand that followed last year's oil price collapse.