Oboe, 13, 44, 139, 177. Ockeghem, 177. Octave (Greek system), 86. Opera, 178, 206, 208, 210. Organ, 33, 94, 132. Organ pedals, 134. Organs (portable), 134. Organum, 128. Orientalism, 151, 173, 204. Osiris, 8, 47. Overture, 189, 216.