The Collaborative International Dictionary of English v.0.48:

doh \doh\ (d[=o]), n.
   the syllable naming the first (tonic) note of any major
   diatonic scale in solmization, usually written do.

   Syn: do, ut.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

doh
    n 1: the syllable naming the first (tonic) note of any major
         scale in solmization [syn: do, doh, ut]