Members of the Organization of Petroleum Exporting Countries unanimously agreed today that the price of oil is going up Jan. 1. The only issure still open is how much.

By the time members of the oil cartel end their semiannual meeting here, probably on Sunday, the available evidence indicates they will have agreed on an increase of about 10 percent.

This amount, if passed through to consumers in the United States, would add about three cents to the price of a gallon of fuel.

Well-informed conference sources said the price probably would go up 5 percent on Jan. 1 and then be raised in several smaller increments after that.

The current price for a 42-gallon barrel of Arabian light crude-the "bench" price against which all oil prices are set-is now $12.70, where it has been frozen since mid-1977. An increase of 10 percent would add about $4.5 billion to the U.S. trade deficit if the current level of oil imports is maintained.

The tone of the meeting here was set in opening remarks by OPEC Secretary General Ali Jaidah, who said the oil producing states are the "the victims and not the cause" of Worldwide inflation. The real causes, he said, were the "selfishness" of the industrail states and politics irrelevant to oil.

Mana Said Oteiba, the petroleum minister of the United Arab Emirates who was elected OPEC president today, continued the theme.

"We have been freesing the price of oil. We did this of our own free will. We agreed to freeze the price of oil for two years because we want to play a constructive role in the world economy," he said. But he said the fall of the dollar and the rising prices of goods imported by OPEC states were "affecting our revenues and investments."

That was the basic message of this meeting-prices must go up and the increase is justified, but they should not go up so much that a new round of inflation will begin and the dollar will be buffeted anew. Most OPEC nations now apparently recognize that they could ultimately suffer from their own pricing actions, especially since most of their reserves are in dollars and their investments are increasingly tied up in the United States.

Demands from some Arab oil states, such as Libya and Iraq, that OPEC jettison the dollar as the currency for determining prices apparently found little support. Conference participants said they spent little time on this proposal and emphasized their desire to balance what they said were their legitimate needs with concern for the value of the dollar and the stability of the world economy.

By the time the delegates adjourned tonight to attend a dinner, only five of the 13 countries had given their views on the price question, OPEC officials said.

Saudi Arabia, the world's biggest oil exporter and the most powerful member of OPEC, was not among them.The chief Saudi delegate, Sheik Ahmed Zaki Yamani, has said that the Saudis want to limit the price increase to 5 percent but a clear majority appeared to prefer a bigger boost than that.

Delegates said one major issue to be settled after the first day was how to phase or stagger the imposition of the increase so as to reduce its economic and political impact on the consuming nations.

"We want to increase the price quarterly," Venezuelan Oil Minister Valentin Hernandez-Acosta said in an interview. "In that way the industrialized countries are going to know in advance what is going to happen to the price of oil."

He said he expected "at least" a 10 per cent increase on the average over 1979-that is, less than 10 per cent at first and more than that by the end of the year.