on-looker

from The Collaborative International Dictionary of English v.0.48

On-looker \On"-look`er\, n.
   A person who observes an event; a looker-on; a spectator.
   [1913 Webster]

from Moby Thesaurus II by Grady Ward, 1.0

36 Moby Thesaurus words for "onlooker":
      TV-viewer, abutter, adjoiner, beholder, bird-watcher, borderer,
      bystander, drugstore cowboy, eyewitness, gaper, gazer, gazer-on,
      girl-watcher, goggler, immediate neighbor, kibitzer, looker,
      looker-on, neighbor, neighborer, observer, ogler, perceiver,
      percipient, seer, sidewalk superintendent, spectator, spectatress,
      spectatrix, tangent, televiewer, television-viewer, video-gazer,
      viewer, watcher, witness