WordNet (r) 3.0 (2006):

orchestrator
    n 1: an arranger who writes for orchestras

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "orchestrator":
   arranger, ballad maker, balladeer, balladist, balladmonger,
   composer, contrapuntist, ethnomusicologist, harmonist, harmonizer,
   hymnist, hymnographer, hymnologist, librettist, madrigalist,
   melodist, melodizer, musicographer, musicologist, scorer,
   song writer, songsmith, symphonist, tone poet, tunesmith