The Collaborative International Dictionary of English v.0.48:

Oto- \O"to-\ [Gr. o'y^s, 'wto`s, the ear.]
   A combining form denoting relation to the ear, or situation
   near the ear or in the ear.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Oto
    n 1: a member of the Siouan people inhabiting the valleys of the
         Platte and Missouri rivers in Nebraska [syn: Oto, Otoe]
    2: a dialect of the Chiwere language spoken by the Oto [syn:
       Oto, Otoe]