The United States is spearheading an international campaign to reverse the ineffectiveness that has paralyzed the 31-member Organization of American States and to make it a more efficient force for resolving disputes and promoting economic progress among Western Hemisphere nations.

The effort began to take shape earlier this month during the annual OAS assembly in Cartagena, Colombia. Prodded in part by Secretary of State George P. Shultz's exhortation for the OAS to play a greater role in advancing Latin America's "surge toward democracy," delegates made some little-noticed changes in the OAS charter aimed at strengthening the organization's ability to deal with regional border disputes and to combat narcotics trafficking.

They also directed OAS Secretary General Jose Baena Soares to recruit a team of experts from North and South America to study possible joint efforts aimed at solving the region's myriad economic problems, including the staggering foreign debt that has brought many Latin countries to the brink of bankruptcy and fanned domestic political unrest.

"They are very tentative and minor first steps, but the OAS has become such an irrelevant diplomatic backwater that any boat-rocking that might keep it from falling into total disuse is to be welcomed and applauded," said one former high-ranking U.S. diplomat who has had extensive experience with the organization.

The former diplomat, who asked not be identified, added that substantial follow-up to the Cartegena actions will be needed, including priority attention from the Reagan administration.

He and current U.S. officials said the principal credit for the Cartegena changes is due to Richard T. McCormack, who became U.S. ambassador to the OAS this year. McCormack got the OAS post largely as the result of an internal State Department shake-up intended partly to shunt him to the sidelines of major policy decisions.

McCormack, a former aide to conservative Sen. Jesse Helms (R-N.C.), came to State as assistant secretary for economic affairs. He failed to have much impact in that job, however, because the department's career diplomats were suspicious of his ties to Helms and because Shultz was known to be unimpressed by his economic expertise.

A year ago, when Shultz decided on a major reshuffling of ambassadorial and policy-making posts, McCormack was among those that the secretary wanted to push out. Strong protests from Republican conservatives about an alleged "ideological purge" led Shultz to make some compromises, and McCormack instead was moved to the OAS ambassadorship.

The OAS, founded early in the century as the Pan American Union and upgraded to its present status following World War II, never has lived up to original hopes that it would be a major arbiter of hemispheric disputes and an instrument of collective defense against outside interference. But as recently as the 1960s, it was regarded by the United States as a major forum for hemispheric diplomacy and thus as an important factor in U.S. policy-making.

The organization served as the springboard from which President John F. Kennedy launched the Alliance for Progress and provided the United States with a mechanism for excluding Cuba from the inter-American system and giving a seal of hemispheric approval to the 1965 U.S. intervention in the Dominican Republic.

In those days, the U.S. ambassador to the OAS -- a post held by such figures on the diplomatic scene as Elsworth Bunker and Sol M. Linowitz -- had substantial influence.

But during the 1970s, a number of factors -- increasing factionalism among the membership, lessened U.S. interest in Latin America and cronyism and infighting within the organization's swollen bureaucracy -- put the OAS on a course that led to its present reputation for being little more than an irrelevant debating society.

It has shrunk from dealing with the tensions in Central America. And when it was confronted with the 1982 Falklands war between Britain and Argentina, the organization played almost no part in seeking a solution.

The current modest U.S. effort to change that condition is due in part to Shultz's concern over the debt problem and the threat it poses to the movement to democracy in many Latin countries. And as one State Department official privately noted, it also is due to McCormack's determination to prove to the foreign policy establishment that he is not a "mindless conservative ideologue" but someone capable of making a mark in diplomacy.

In pursuing that goal, McCormack was aided by some accidents of timing. One was the departure from the secretary general's post of Alejandro Orfila, a flamboyant but controversial figure who was mistrusted by many members, and his replacement by Baena Soares, a low-key but respected veteran of Brazil's diplomatic service.

The other was the fact that McCormack became ambassador just as the United States was about to assume its once-a-decade turn as chairman of the OAS permanent council. That put him in charge of planning for the Cartegena meeting and, with Shultz's backing, he seized the opportunity to lobby the other members for the changes.

McCormack said he found other members receptive to the idea of strengthening the OAS. The result, after months of negotiation, was the series of charter changes adopted at Cartegena. Latin American diplomatic sources added that many member countries, which had balked at changes that would have increased Orfila's power, now are willing to give this flexibility to Baena Soares and to rank the OAS more highly in their foreign policy considerations.

That was perhaps most evident in McCormack's success in persuading many Latin members, whose failure in recent years to meet their OAS financial obligations has brought the organization to the verge of insolvency, to start paying their past-due bills.

Other developments at Cartegena included the scheduling of a major international conference on narcotics trafficking in Brazil in April, the start of a drug-awareness program and an agreement for a new try at resolving long-standing and emotional disputes along the Venezuela-Guyana and Guatemala-Belize borders. Perhaps most important in terms of U.S. interest, a far-reaching study will be made of the region's economic problems.

Most of these steps are what the former diplomat called "nibbling at the edges."

The point is conceded by McCormack, who said: "We're dealing with an organization that has been leading a demoralized, hand-to-mouth existence for years. I think we've taken some pragmatic first steps toward bringing it back. But there's no question that there is a long way to go."