If the OPEC minister closest to the United States, a stout Venezuelan named Humberto Calderon Berti, could walk into a North American gas station today and address himself to the enraged working man or woman whose automobile was about to consume yet another $15.75, this is what Calderon believes he would say:

"We have 30 countries, industrialized countries having housing, food, clothing, access to education. They have everything they need. On the other side, we have 120 countries. Some of them don't have anything. They don't have access to education. They don't have housing facilities. Food -- very limited quantities of food. . ."

He would raise one thumb, and shake it a little toward his listener, to make his point. "Before 1970, the oil companies paid whatever they wanted for this oil. They established the price. But the industrialized countries didn't ask us how much we wanted to pay for their cars, their refrigerators, the things we imported from them.

"What's really happening now?We're recovering what we lost before. Nothing else."

In terms of direct impact on the average citizen of the United States, the most powerful man in South America may be this round-faced 39-year-old petroleum engineer who worked part time 10 years ago solving scientific problems for what he says was $1.30 an hour at the University of Tulsa.

As minister of energy and mines for Venezuela, Calderon has become part of an extraordinary presence -- the petrodiplomats of the Third World. Venezuela was a founder of the Organization of Petroleum Exporting Countries and the only other Latin American member is Ecuador.

Calderon sits down at meetings 10,000 miles away, and the world economy holds its breath. His nation's oil, or the promise of it, makes high officials of the industrialized world bring offers of wealth and technology to the conference table in his Caracas office. By many reports, it was his steady politicking and personal visits that finally overcame internal rancor and the Iranian-Iraqi war to bring the OPEC countries together for the recent Bali meeting at which oil prices were raised to as much as $40 per barrel.

Calderon is a highly popular character in Venezuelan politics -- charming, given to chortling belly laughs, usually ready after meetings with a few words for local reporters when other officials brush by them.

"If you see the modern world, let's say from World War II to now, the last 35 or 40 years," Calderon said in accented but fluent English in an interview, "the most powerful instrument belonging to the developing countries is OPEC. What would be the difference between Venezuela and Nicaragua if we didn't have oil?

"But we have to ask ourselves, are we going to have oil forever, all the OPEC countries? And the answer is no. In the long term . . . we are going to face the same problems all the developing countries are facing now.

That is why we have insisted that we have to use OPEC not only for our own interests.

"We believe out struggle is the struggle of all the developing countries. And it's the only instrument, the only tool the developing countries have, with enough power to discuss with the developed countries."

In his speeches, and presumably in the private sessions where OPEC ministers debate, Calderon is pushing the idea that OPEC must embrace much more than the business of selling oil. As it emerges from its 200th year, battle-scarred but still intact, OPEC has accumulated an international weight that Calderon says must now be used to defend the prices of raw materials exported from the Third World, to force industrialized countries into a more effective sharing of their technology, and to help finance development.

Hesitant beginnings of Calderon's vision are taking shape. An $800 million OPEC fund, established in 1976, grants loans to developing countries to offset their increased energy costs.

A Mexican-Venezuelan aid program promises regular oil supply to Caribbean and Central American countries, and then lends them part of the money to pay for it. In El Salvador, for example, Calderon has just concluded renewed agreements that will deliver 8,000 barrels of Venezuelan oil per day and lend the Salvadorans $31 million.

When OPEC's members decides recently that international press coverage of their activities played too predictably to the viewpoint of developed countries, they began broadcasting a viewpoint of their own. "I think the image [of OPEC], not only in the United States, but in the rest of the world, is not so good," Calderon said. "That is the basic reason we proposed a news agency. That was proposed by Venezuela."

He still defines Venezuela, which has the highest gross national product per capita in South America, as a developing country. "It's not a matter of dividing income into populations," he said. "That's not the problem. The problem is, how can you distribute the income?"

That addresses the most recurrent criticism of Venezuela today -- that it is a society in which the oil money has been lavished on inefficient projects and conspicuous consumption while 30 percent of its people still live in poverty and deepening anger.

"We haven't been able to promote the development from the oil," he said. "We were not prepared. We don't have enough educated people to handle the money in a useful way. We have proven that more dollars do not mean we are richer."

He blames that partly on the national educational system. "Remember that Venezuela was a very underdeveloped country 30 years ago," he said. But also at fault, Calderon adds, is what he once called "the most sophisticated way to subdue developing countries today" -- the refusal of developed countries, as he sees it, to share on a large enough scale the technology the Third World needs.

"Usually, when the private enterprises come to these countries, they try to sell what they have," Calderon said. "They don't care if they are selling what the countries need. . . . We have a hospital. Let's take this example. Very simple. In a very hot area of Venezuela, the equipment was sold with heating facilities."

Surely the buyer shares some responsibility for that, it is suggested.

"Sometimes we don't know enough to know what to buy," Calderon said. "Developed countries have to help the others to buy what they need. And that's not impossible. That's very possible."

Calderon wears his power with mixed emotions in public. He hates the label "diplomat," hoots at the suggestion that his position has made him one of the world's most powerful government officials, and likes to mention that he helps his children with their homework every night. But there emerges, now and then, some sense of pleasure at his own weighty role.

"I don't believe there are too many people who can go to El Salvador, shake hands with [President Jose] Napoleon Duarte as a friend, as a personal friend, have lunch with him, and go to Nicaragua, and have dinner with [junta member] Daniel Ortega." Calderon laughed."I visited Costa Rica, and President [Rodrigo] Carazo was waiting for me at the airport, because he wanted to see me. And we met almost four hours. And you know where we met? In my room."