Oscar Arias, the president of Costa Rica, received the Nobel Peace Prize in Oslo on Dec. 10. Now he's got to earn it.

The Central American peace plan that has his name is in bad trouble. It will go down unless he shows some of the stuff that enabled him to get his four brother Central Americans to sign on the dotted line last August.

The Reagan administration, of course, is hoping that this first venture in home-made Latin American diplomacy sinks without a trace.

Arias doesn't have much time. On Jan. 15, the five leaders meet in Costa Rica to say whether the peace plan is working. A showdown on contra aid in the U.S. House is scheduled Feb. 3.

Correspondent Julia Preston reports from Managua that the international peace monitors who are meeting to pass preliminary judgment on compliance by participants are falling on each other with bitter new accusations.

No ceasefire between the Sandinistas and contras is in place or in sight.

The principal mediator in the Nicaraguan ceasefire, Cardinal Miguel Obando y Bravo, complains that the Sandinistas are dragging their feet.

El Salvador's president, Jose Napoleon Duarte, took the pact signed in Guatemala City as a way of further demonstrating his fatal subservience to the military and has turned it into a death squad's dreams. He interpreted a section on amnesty in the accord to mean that he should release a pair of National Guardsmen convicted in the 1981 murders of two American advisers, David Perlman and Michael Hammer. The State Department pronounced itself "outraged" and "appalled."

In the session's last hours, Congress voted to grant the contras another $8 million in aid, guaranteeing their obduracy. "It was," said one sardonic observer, "a footrace between conscience and the Christmas recess, and conscience had no chance."

Honduras, which bravely announced in December it would cooperate and close the contra camps within its borders, has had second thoughts, induced, at least partly, by the timely acquisition of some new U.S. fighter planes.

The prospects for endless war, in fact, have never looked better.

House Speaker Jim Wright (D-Tex.) -- who was patron of the plan, organized the Democrats in support and fought the White House every inch of the way -- has gone home to Texas, weary, it is said, of warring Latins. A letter he wrote to Secretary of State George P. Shultz, inviting his close attention in this critical phase, has gone unanswered.

Shultz, accompanied by White House chief of staff Howard H. Baker Jr., spent many hours with Wright and arranged for a show of administration support of the Arias plan. Now he has bowed off the scene. He is devoting himself to a more promising problem, the improvement of Soviet-U.S. relations. Management of the affair has reverted to Elliott Abrams, assistant secretary of state for inter-American affairs, a super-zealot for the contras in the Ollie North class.

The situation is unfolding in a manner that gladdens the hearts of Abrams and company. Nicaragua is being set up as the saboteur.

A two-man Christmas week peace mission to Santo Domingo, where Obando was negotiating with the contras, failed miserably. Paul Reichler, the young Washington lawyer who has represented the Sandinistas for the past eight years, and Hans-Jurgen Wischnewski, a veteran West German Socialist troubleshooter, were dispatched to the bargaining site by the Sandinistas. They were armed with an 11-point ceasefire proposal they thought the contras could not refuse. The contras, professing a nationalism that could not brook talking about their country's affairs with foreigners, refused to meet them.

Nicaraguan President Daniel Ortega took a big chance in naming the cardinal, the Sandinistas' worst enemy, as the go-between. He thought that Obando would bring credibility to the process. So far, there has been no process. Catholic prelates, who are against contra aid, have remained quiet, because Wright, in one of his many initiatives, consulted the papal nuncio and U.S. bishops feel the matter is in the hands of Rome.

Arias is being told that it is the ninth inning with two outs, and he better get out his bat for this month's meeting. The most he can hope for at this point is that the presidents will vote to extend the deadline for compliance and insist that, however it looks, peace has a chance.

The most he may have going for him is the reluctance of individual Central American leaders to be the one to pull the plug on a plan that has brought a little luster to the region.

It will take everything Arias has to persuade the four countries, three of which are almost totally dependent on the United States, that they can stand up for peace and survive the wrath of Uncle Sam and Elliott Abrams.