The Collaborative International Dictionary of English v.0.48:

Oca \O"ca\, n. [Sp.] (Bot.)
   A Peruvian name for certain species of Oxalis (Oxalis
   crenata, and Oxalis tuberosa) which bear edible tubers.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Oxalis crenata
    n 1: South American wood sorrel cultivated for its edible tubers
         [syn: oca, oka, Oxalis tuberosa, Oxalis crenata]