The Organization of Petroleum Exporting Countries probably will increase 1979 oil prices more than had been expected earlier.

That is the view of a growing number of OPEC watchers as well as OPEC insiders.

During the summer, the oil-producing states seemed willing to settle for an increase in 1979 oil prices as small as 5 percent, even though it would be the first OPEC-wide boost in two years.

But in recent weeks, predictions of the coming increase have been creeping up. Publicly voiced industry estimates currently are in a range of 5 to 8 percent, but private projections have moved to the upper end of the range, with some going far higher.

"It looks like 10 percent," says a Middle East specialist with an international oil company that handles vast quantities of OPEC oil.

Much can happen over the remaining seven weeks before the oil ministers of the 13 member states of OPEC meet in Abu Dhabi to decide on the 1979 benchmark quote. For example, there is considerable debate within OPEC on whether the 1979 increase - whatever its size - should be made effective in full on New Year's Day or spread over the year in a series of small boosts.