The Collaborative International Dictionary of English v.0.48:

Oarlock \Oar"lock`\ ([=o]r"l[o^]k`), n. (Naut.),
   The notch, fork, or other device on the gunwale of a boat, in
   which the oar rests, and that holds the oar in place and acts
   as a fulcrum for rowing. Certain oarlocks are designed for
   use with oars having attached swivels, which insert into the
   oarlock and provide a firm pivot. Same as Rowlock[1].

   Syn: peg, pin, thole, tholepin, rowlock.
        [1913 Webster + WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

oarlock
    n 1: a holder attached to the gunwale of a boat that holds the
         oar in place and acts as a fulcrum for rowing [syn: peg,
         pin, thole, tholepin, rowlock, oarlock]