Not long ago the OPEC gathering in Vienna next week would have had the world waiting breathlessly for any sign of the group's plans for oil supply. Not anymore.

Because the Organization of the Petroleum Exporting Countries produces about 40% of the world's crude oil, its decisions on collective output can move markets. But how much attention do traders pay? One Switzerland-based physical oil trader responded bluntly: "We don't."

Traders commented anonymously because the terms of their contracts prohibit them from speaking to the media.

Five years ago, the trader said, the twice-yearly meeting was more important.

In the chaotic aftermath of the financial crisis, the price of Brent crude plunged from record highs above $147 a barrel to below $39 a barrel in just six months. The market only began to recover after OPEC decided to cut around 14% of its total output at an extraordinary meeting in December 2008. OPEC has now maintained its production target unchanged for two years.

Today, the only OPEC country that really matters is Saudi Arabia, the trader said. It alone has the ability to ramp up or scale back its production to balance the market. That could change if Iran or Libya come back fully to the global production arena, the trader said.

In recent months oil worker strikes have repeatedly disrupted supply from Libya. Iran is engaged in talks to loosen sanctions imposed against much of its oil.

Supply disruptions in individual countries have more effect on prices than OPEC's official decisions, said a London-based trader. Countries such as Libya and Nigeria, which have both experienced major disruptions this year, are closely watched for signs of stability or distress.

One of the reasons for OPEC's waning importance is U.S. shale oil. With its big industry and thirsty cars, the U.S. has long been the world's largest consumer of crude oil, much of it from OPEC. But in the last five years U.S. domestic production has shot up, meaning lower demand for imported crude.

Oil is still shipped in huge quantities to emerging economies like China—but there is more to go around.

The change isn't only being felt in physical trading—the buying and selling of shiploads of oil—but also in the market for oil futures.

The importance of OPEC in recent years has declined, especially from a futures trading perspective, said an oil futures trader based in the U.K. "Longer term, we know where OPEC spare capacity is and we have a good idea where current production is," the trader said.

A fourth trader said that there are still times when OPEC's influence is key:

In 2011, Libyan production dropped and it wasn't clear how suppliers would respond. "So we waited [to see] what was the organization going to do to try and rebalance the market," the trader said.

But he added: "When the market is balanced why should they do anything?"

Write to Cassie Werber at