Octave: the range between two frequencies whose ratio is 2:1.

Organ of Corti: a complex structure in the cochlea of the middle ear, supported by the basilar membrane and containing the hair cells.

Oscillation: a to-and-fro motion; a fluctuation of a quantity or value about a mean.

Oscilloscope: a device for displaying oscillatory signals on a cathode-ray screen.

Ossicles: the three small bones of the middle ear, which connect the eardrum with the oval window in the cochlea.

Otitis media: inflammation and/or infection of the middle ear leading to build-up of fluid and blockage of the Eustachian tube and may result in temporary or permanent hearing loss (also sometimes called ‘glue ear’).

Otoacoustic emission: sounds emitted by the eardrum and detected by a miniature microphone in the ear canal arising from activity in the cochlea in response to the stimulus of external sound.

Outer ear: the outer part of the hearing mechanism, which collects and guides airborne sound down the ear canal to the eardrum.

Output impedance: the impedance of a device measured at its output.

Oval window: diaphragm connecting the cochlea to the middle ear.

Overdamping: (1) an amount of damping, in excess of critical, which is sufficient to prevent oscillation in a mass-spring system; (2) producing a damping ratio greater than one.

Overload: a situation in which a component or system is used beyond its range of linearity.

Overload indicator: a device, which indicates when an instrument is likely to read incorrectly because it is being overloaded.

Overtone: a higher (i.e. not the lowest) harmonic or natural frequency of a vibrating system.