To form the possessive of any acronym, including those ending in S, add an apostrophe and an s. Therefore, the possessive form of UN is UN’s, and the possessive of OAS is OAS’s.

By contrast, to make an acronym plural add an s without an apostrophe.

It is best to avoid using too many acronyms in formal academic prose. An acronym is appropriate when you need to repeatedly refer to the name of an organization, especially one with a long name. When this is the case, spell out the name of the organization when you first mention it and then give the acronym in parentheses. Use the acronym in subsequent mentions of the organization. 

In November, the American Council on the Teaching of Foreign Languages (ACTFL) will hold its annual convention. Many of ACTFL’s thirteen thousand members are expected to attend.