OAS official documents published since 2000 (there are a few older items) can be searched on the OAS Document Search interface. Specific types of documents can be found on the following pages:
The Van Pelt Library has collected publications from different bodies of the Organization of American States and also documents from its predecessor, the Pan American Union. In Franklin, there a number of possible searches:
"Organization of American States" as author/creator
"Columbus Memorial Library" as author/creator
"Pan American Union. General Legal Division" as author/creator
"Organization of American States. General Secretariat" as author/creator
"Inter-American Juridical Committee" as author/creator
"Inter-American Economic and Social Council" as author/creator
"Inter-American Commission on Human Rights" as author/creator
"Pan American Union. Dept. of Social Affairs" as author/creator
"Inter-American Statistical Institute" as author/creator
"Pan American Union" as author/creator
"Pan American Union. Division of Intellectual Cooperation" as author/creator
"Organization of American States" as a subject heading keyword
"Pan American Union" as a subject heading keyword
The Biddle Law Library also has documents related to the Organization of American States, which can be found through the LOLA Catalog:
"Organization of American States" as author
"Pan American Union" as author
"Organization of American States" as a subject heading
Use WorldCat to find OAS publications and books about the OAS that are held in libraries around the world, including the OAS Columbus Library in Washington, DC.
The OAS has a complex system of classification of documents, that includes a common designation (OEA), followed by a subseries (A-Z, with some letters still unassigned), and further by Roman numerals and then Arabic figures.
The series are as follow:
OEA/Ser.A: Multilateral Agreements, Conventions, and Treaties
OEA/Ser.B: Agreements and Arrangements to Which the OAS Is a Party
OEA/Ser.C: Final Acts of Inter-American Conferences and Meetings
OEA/Ser.D: Administrative Reports and Documents of the General Secretariat of the OAS (Formerly the Pan American Union)
OEA/Ser.E: Documents, Journals, and Proceedings of the Inter-American Conference (Formerly the International Conference of American States)
OEA/Ser.F: Documents, Journals, and Proceedings of the Meeting of Consultation of Ministers of Foreign Affairs
OEA/Ser.G: Documents of the Permanent Council (Formerly the Council) of the OAS and Its Committees
OEA/Ser.H: Documents of the Inter-American Economic and Social Council (CIES), Its Permanent Executive Committee (CEPCIES), Its Other Committees and the Special Committee on Trade (CEC)
OEA/Ser.I: Documents and Proceedings of the Inter ·American Council of Jurists (IACJ) and Its Permanent Committee
OEA/Ser.J: Documents at the Inter-American Council for Education, Science, and Culture (CIECC), Its Permanent Executive Committee (CEPCIECC) and Its Other Committees (Formerly the InterAmerican Cultural Council (CIC) and Its Permanent Committee, the Committee for Cultural Action (CAC)
OEA/Ser.K: Documents of the Inter-American Specialized Conferences and Other Inter-American Conferences and Meetings
OEA/Ser.L: Documents of the Inter-American Specialized Organizations, the Inter -American Commission on Human Rights, and the Inter-American Entities
OEA/Ser.M: Bilateral and Regional Agreements Deposited at the General Secretariat of the QAS (Formerly the Pan American Union)
OEA/Ser.N: Documents, Journals, and Proceedings of the Inter-American Conference for the Maintenance of Continental Peace and Security
OEA/Ser.O: Not assigned as of August 1995
OEA/Ser.P: Documents, Journals, and Proceedings of the General Assembly of the OAS
OEA/Ser.Q: Documents of the Inter-American Juridical Committee
OEA/Ser.R: Documents of the Administrative Tribunal of the OAS
OEA/Ser.S: Documents of the Board of External Auditors to Examine the Accounts of the General Secretariat
OEA/Ser.T: Documents of Ad Hoc Bodies Established by Organs of the OAS
OEA/Ser.U: Documents of the Meetings between the Organization of American States and the United Nations
OEA/Ser.V: Not assigned as of August 1995
OEA/Ser.W: Documents of the Inter-American Council for Integral Development
OEA/Ser.X: Collections of OAS Official Records
OEA/Ser.Y: Guides to the Use of the OAS Official Records Series
OEA/Ser.Z: Indexes of the OAS Official Records Series
For updated information on the series, see Manual for OAS Proofreaders.
For detailed information on the subdivisions of each series, see Official records series of the Organization of American States: guide, outline, and expanded tables. (does not include information on Series N-W)
The Classification manual for the OAS official series, available at the Van Pelt Library in both Spanish and English, might also be useful.
Wikipedia: Leo Stanton Rowe, Professor of Poltical Science, U. Pennsylvania, 1896-1917; Director General, Pan-American Union, 1920-1946.
Classification Manual for the OAS Official Records Series by 
Manual de Clasificación para la Serie de Documentos Oficiales de la OEA by 