The price of crude oil fell worldwide today with the possibility that a plan by Saudi Arabia to sell discounted oil to selected buyers would lead to a price war.

Saudi Arabia's oil minister, Sheik Ahmed Zaki Yamani, confirmed Friday at a seminar at Oxford University, according to participants, that his country would abandon its longtime policy of adhering to the official prices set by the Organization of Petroleum Exporting Countries (OPEC).

The Saudis have been almost alone among OPEC's members in adhering to the official price. As a result, buyers have looked for cheaper oil elsewhere, and Saudi Arabia's output has fallen to a 20-year low.

Sheik Yamani has denied press reports suggesting that he had predicted that oil prices would plunge to between $15 and $18 a barrel, OPEC's official news agency reported today. "Quite the contrary, I predict prices will firm up in the coming few months," he was quoted as saying. "I said at the Oxford energy seminar that if non-OPEC producers do not cooperate with OPEC in stabilizing the market, and we in the organization do not discipline ourselves, then I expect there will be a price war."

OPEC countries often have predicted a price war in calling on other producers to curtail production, but Saudi Arabia's discount plan adds force to the threat.

North Sea Brent crude oil, one of the most widely traded varieties, was quoted today at $26.35 a barrel for November delivery, down 40 cents from Friday. On the New York Mercantile Exchange, West Texas intermediate crude for delivery in November was trading at $27.25, down from $27.31 Friday. OPEC's present official price is $28 a barrel.

Sheik Yamani did not discuss details of the Saudi discounts, but said they would involve selling oil on a "netback" system. Under that system, used by some other OPEC members, the price is based on the current market value of the products that can be refined from the oil.

The Middle East Economic Survey, a newsletter noted for its contacts with the Saudis, reported today that the discounts would cover about 800,000 barrels per day. Saudi output in recent months has totaled between 2 million and 2.5 million barrels a day, but the country signaled in July that it planned to raise production closer to its OPEC quota, 4.3 million barrels.

The companies to which the discounted oil will be sold are Exxon Corp., Texaco Inc. and Mobil Corp., industry sources say. Officials of those three companies declined to comment on the reports, according to the Associated Press. Chevron Corp., another major American oil company, said last week that it was interested in netback purchases from the Saudis but had not yet begun talks on the issue.

"The deals are intended as a warning signal to other oil exporting countries, both OPEC and non-OPEC, that Saudi Arabia can no longer be taken for granted as the sole buttress of world oil prices," the newsletter said.

Oil ministers of the 13 OPEC countries are to meet in Vienna on Oct. 3, and Saudi Arabia's break with the group's official price is expected to impress other members of the danger of an all-out price war that might result unless OPEC members follow the pricing rules.

Observers say the Saudis appear eager to minimize the effect of this discounting. "I don't think they want to wreck the market," said John Lichtblau, president of the Petroleum Industry Research Foundation in New York.

Saudi Arabia plans to limit the sale of its discounted oil to major oil companies that can use the oil within their own refining and marketing networks, analysts say. They say such sales would be much less disruptive than dumping cheap oil onto the open market.