OBI is a 3 letter word that starts with O.

1. \O"bi\, n. [Jap.]
   A sash, esp. the long broad sash of soft material worn by
   women.
   
   

2. \O"bi\, n. [Prob. of African origin.]
   1. A species of sorcery, probably of African origin,
      practiced among the negroes of the West Indies. [Written
      also {obe} and {obeah}.] --De Quincey. --B. Edwards.
   
   
   
   2. A charm or fetich. [West Indies] --B. Edwards.