Possible Clues:

• Montevideo is its capital: Abbr.
• Nation NE of the Rio de la Plata
• OAS member
• S. Amer. country
• S.A. nation
• O.A.S. member: Abbr.
• South Amer. country
• Land south of Braz.
• Country east of Arg.
• Pres. Vázquez's nation