The Organization of American States, overriding U.S. objections, voted yesterday to hold a special meeting of foreign ministers next Monday to consider Argentina's call for assistance from Western Hemisphere countries against Britain in the Falkland Islands crisis.

The vote, which could lead to an Argentine request that OAS members use military force or other sanctions against Britain, was 18 to 0, with the United States, Colombia and Trinidad abstaining.

It came as President Reagan appealed to Britain and Argentina to avoid an armed conflict and give Secretary of State Alexander M. Haig Jr. more time to pursue a peaceful settlement after he meets here tomorrow with British Foreign Secretary Francis Pym.

During a brief talk with reporters at the White House, Reagan said it would have been more "advantageous" if the OAS had put off acting on Argentina's request to consider action under the 1947 Inter-American Treaty of Reciprocal Assistance. The United States is among 21 active OAS members that are signatories to the accord, commonly known as the Rio Treaty.

Asked which side the United States would support if the British-Argentine dispute explodes into warfare, Reagan replied, "That would be a terrible thing to say in the midst of all the delicate negotiations."

The United States is known to have made clear to the Argentines that in a showdown it would side with Britain. But the administration wants to avoid an open breach with Argentina, to enable Haig to continue his mediation effort and to preserve the ties it has cultivated with President Leopoldo Galtieri's government.

This desire to avoid having to choose sides was implicit in Reagan's comment and in the position outlined to the OAS by Ambassador J. William Middendorf II.

In his unsuccessful appeal for the hemispheric body to put off taking the first step toward invoking the Rio Treaty, Middendorf questioned whether the Argentine request, which implies a recourse to military remedies, was "necessary or appropriate" while peacemaking efforts are continuing and said:

"The United States is deeply disturbed by the implications of the action. At a time when Secretary Haig is engaged in efforts to promote a peaceful solution, it seems to my government particularly inappropriate to seek a resolution within the framework of the Rio Treaty."

Despite Haig's strenuous efforts earlier to dissuade the Galtieri government from resorting to the Rio Treaty, the Argentines went ahead with their request for an extraordinary OAS meeting to consider whether the British fleet steaming toward the Falklands poses a threat of aggression that calls for collective security measures.

In requesting the foreign ministers' meeting, though, the Argentines made clear that they are not yet asking for specific steps against Britain.

Instead, the Argentine move generally was viewed as a tactical maneuver to pressure the United States and counter a U.N. Security Council resolution calling for the withdrawal of Argentine forces from the Falklands by putting most Latin American nations on record as supporting Argentina's claim to sovereignty over the islands.

Argentina's OAS ambassador, Raul Quijano, called the British threat to retake the Falklands by force a reversion to "gunboat diplomacy" and argued that the British armada poses a threat "not only to Argentina, but to the entire American continent and endangers the region's peace and security."

But Quijano also stressed that his government, which made its OAS appeal Monday night shortly after Haig left Buenos Aires after four days of talks, wants the secretary to continue his mediation attempt. "Argentina values those efforts," he said.

Haig, who arrived here early yesterday, reported to Reagan at the White House, and administration sources said later that the U.S. mediating effort had been put on hold until Pym arrives here with proposals from Prime Minister Margaret Thatcher's government.

Haig had been shuttling between Washington, London and Buenos Aires since April 6, and one official said yesterday:

"He tried very hard, but he wasn't getting anywhere in bridging the gap between the two sides, so it was decided that it would be wiser to bring him back and let the contestants think about whether they really want a confrontation that's getting closer every day."

"So far," the official added, "Argentina's the only one that's really been putting forward any proposals, although they've fallen far short of what Britain will accept. Now it's time to see what the British have to say, and for that, we have to wait for Pym."

U.S. officials hope Pym will bring ideas that will permit resumption of the stalled peace effort before the British warships' expected arrival in the Falklands vicinity this weekend and before the OAS is forced to consider a possible Argentine demand for specific measures against Britain.

If that demand is made, there is no doubt that the United States, however great its reluctance to antagonize Argentina, will state that its obligations to Britain have precedence and that Washington is not obligated to act under the Rio Treaty because Argentina, in forcibly occupying the Falklands, struck the first blow.