At the very end of the road that Oscar Arias wants the hemisphere to go down as an alternative to American military intervention in Nicaragua lies, crazily enough, American military intervention in Nicaragua.

No fooling. It's a very long shot. I know of no one who predicts it will come. The Costa Rican president hopes there will never be a need to go that far down the alternative road. He hopes Nicaragua will bow to the region's and hemisphere's moral, political and economic pressures to democratize -- the pressures that are currently being applied and others that might be organized if the Sandinistas drag their feet.

But he has stated in at least a few quarters that if the Sandinistas finally do not comply with the commitments they have undertaken in the Central America peace plan, then he will consider invoking the hemisphere's basic security charter, the Rio Treaty, and asking for American intervention. He does not think the contras are up to the job.

Part of his openness to this option is Latin legalism. Part may arise from a traditional Latin feeling that, if things really get bad enough -- Arias mentions the Dominican Republic and Grenada -- the United States will invade and take care of a problem that the Latins know is serious but cannot handle themselves, encumbered as they are by military inadequacy and their political scruples against intervention.

If something like this ever happened, the intervention would be in the direct form of American forces rather than in the indirect form of sponsorship of Nicaraguan rebels, and it would be triggered by a formal treaty request of the hemisphere's political organization, the Organization of American States, rather than by a decision taken by the United States alone. The first circumstance would transform the military scene, the second the political.

I raise the matter because it has obviously occurred to the Sandinistas to start repairing their position in the OAS, which in 1979 did them the immense service of pulling the plug on the Somoza dictatorship only to find them later ignoring the OAS's quid pro quo of democracy and human rights.

The Sandinistas did not seem to fret about the OAS while the United States was out front with the contras. But when Arias got out front with a Latin plan, they could no longer hide behind the built-in Latin antipathy to almost any form of U.S. intervention. This is surely why Daniel Ortega showed up -- dared to show up -- at the current OAS meeting, his first, in Washington. It is why the Nicaraguans circulated a proposal -- Latin pride makes it a non-starter -- to rule out any OAS sanctions not approved by the United Nations.

Would Oscar Arias actually ever go to the extreme of a request for American intervention? Presumably it would require the Sandinistas to abandon the discretion that now marks their policy and to cross a threshold of hemispheric outrage that they now respect. It would require from Arias himself a decision that he had been deceived by the Nicaraguans and held up to dishonor. As I say, a very long shot.

''Some observers,'' George Shultz suggested to the OAS on Tuesday, wonder whether such organizations can tackle the ''highest priority issues.''

Certainly the OAS will help as it can to put into effect any agreement among the Nicaraguan or Central American parties. But in the event of a second Sandinista betrayal, would the OAS ever go along with an Arias request for intervention or, for that matter and perhaps more to the immediate point, with a request for sanctions less severe than a military operation?

That leads you to start counting votes. Here is one savvy count of the key ''Rio group'' of democracies that constitute a tentative ''OAS without the United States": Mexico and Peru would be in opposition, Colombia and Venezuela would be more favorably inclined, Brazil would keep out of the discussion, Argentina and Uruguay would be pivotal, but they have strong, easily mobilized lefts.

Whatever happens, Shultz's point remains valid: ''After what happened in 1979, it would be doubly irresponsible for us {of the OAS} to help reopen the door of democratization in Nicaragua only to allow it to be slammed shut again.''