OPEC oil ministers, trying for the third time this year to unify prices, announced today that a special meeting will be held Oct. 29 in Geneva.

A single basic reference price has eluded the 13-member Organization of Petroleum Exporting Countries since early 1979, when the Iranian revolution and, later, the Iranian-Iraqi war, allowed OPEC's price hawks to break ranks with moderate members and run up their oil charges.

The announcement of another emergency meeting on the issue, following failures to reach agreement in May and August, signals that previous obstacles to a deal may well have fallen away. The OPEC chairman, Indonesian Oil Minister Subroto, told reporters in Jakarta, "We have already more or less formed a consensus to unify prices."

"Unification will be the main part of the discussions," Subroto said. "My opinion is that there will not be much problem concerning differentials -- that has been more or less settled already."

Subroto said that the special conference would be a short one and that any decision it made on prices would be implemented immediately.

Although he declined to say the level at which unity might be achieved, $34 per barrel has been predicted widely.

Such a compromise would require official price reductions by the militant North African producers, Libya and Algeria, as well as by such moderates as Venezuela. In return, Saudi Arabia, OPEC's dominant producer, would increase the price for its Arabian light crude by $2 per barrel.

This may mean a slight rise in the average price of OPEC oil, since the Saudi increase would outweigh the price cuts by smaller producers. But a unified price still should benefit consumers, since it is expected to be accompanied by the promise of a freeze in oil prices until the end of next year.

A Saudi proposal for a $34 per barrel unified price, which included the strong hint of a Saudi production cut, was spurned in both meetings earlier this year, and the Saudi oil minister, Sheik Ahmed Zaki Yamani, was said to be unwilling to attend further special meetings unless agreement on his country's desired unified price and accompanying freeze would be assured.

Venezuela, which sells its oil for $36 per barrel, proved a main obstacle to agreement in August, claiming that domestic political considerations prevented it from lowering prices. Oil accounts for most of Venezuela's export trade and for 72 percent of the state's income.

Venezuela's opposition softened in recent days, however, as world demand for OPEC oil continued to drop.