The Post's editorial of Oct. 7 describing the budgetary situation at the United Nations ignored another international organization that is right here in our own neighborhood.

At present, the United States owes tens of millions of dollars to the Organization of American States. Our failure to pay what we owe is seriously weakening the OAS at the very time it most needs to be strengthened. Most of the other countries in the hemisphere -- despite far more serious economic and financial problems -- have been paying theirshare.

The United States may have felt the need to apply economic pressure to force changes at the U.N., butsuch changes are not needed at the OAS:

The OAS has played an important role in furthering the growth of democracy in the hemisphere. The other day President Reagan declared that "democratic transformation {in the hemisphere} is one of the proudest chapters in human history" -- and he went to the OAS to say it.

The OAS has no Eastern-bloc or anti-Israel forces with which the United States must contend.

OAS programs are not politicized.

The U.S. assessment for the OAS this year is the same dollar amount as it was in 1982 -- actually, it is 25 percent less because of inflation.

Critical hemisphere problems call for an OAS strong enough to play its proper role in advancing peace, security and prosperity in the region. It simply cannot perform that role without the financial support of its major contributor.

J. WILLIAM MIDDENDORF II; JOSEPH JOHN JOVA

Washington The writers were U.S. ambassadors to the OAS.

I read with great interest The Post editorial of Oct. 7 titled ''U.N. Deadbeat.'' On a relative basis, the Organization of American States has an even greater problem in trying to cope with deferred payments and with actual cuts in U.S. contributions below the amounts approved by the OAS General Assembly.

The problems is more severe for the OAS because the United States is the only developed country in the organization. The United Nations, however, can look to many developed nations for support of its work.

In fact, the United States has not fully complied with its financial obligations to the OAS since 1981. The organization carried out the administrative reforms that were requested, including a new tax agreement that the United Nations has not accepted. In the process, the OAS discharged about one-third of its staff. Butthe United States still has notcomplied with its financial obli-gations.

The problem is even more serious than that for the OAS. While member governments were, since 1981, regularly approving real or inflation adjusted increases for the United Nations, the Pan-American Health Organization and the international financial institutions in Washington, these increases were denied to the OAS.

Through inflation alone, the budget of the OAS has been additionally cut by almost 30 percent since 1981. Furthermore, the U.S. Congress is proposing another reduction in the U.S. quota in 1988.

I naturally share the concern expressed in the editorial about the United Nations, but I considered it imperative that I speak out on the problems facing the organization that represents the Americas.

JOAO BAENA SOARES Secretary General Organization of American States Washington