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O
 O/p.  Output.
Objective.  The very first optical element at the front of a lens.
Ocular.  The very last optical element at the back of a lens (the one closest to the detector).
Ohm.  The unit of resistance.  The electrical resistance between two points of a conductor
where a constant difference of potential of 1 Volt applied between these points produces in the
conductor a current of 1 Ampere, the conductor not being the source of any electromotive force.
It can be represented by the formula V = I x R, where: V = Voltage I = Current R = Resistance.
Oscilloscope (also CRO, from Cathode Ray Oscilloscope).  An electronic device that can
display changes of voltage on the vertical scale against time horizontally.  The single most
important piece of test equipment in CCTV.
Overscan.  A video monitor condition in which the raster extends slightly beyond the physical
edges of the CRT screen, cutting off the outer edges of the picture.
Output impedance.  The impedance a device presents to its load.  The impedance measured at
the output terminals of a transducer with the load disconnected and all impressed driving forces
taken as zero.