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Ohm : ($$ \Omega$$) The unit for resistance

Ohm's Law : Named after George Ohm :- V = IR. V = voltage, I = current, R = resistance

OLIVIA : A digital mode using multiple FSK carriers; intended for operation under QRP or weak-signal conditions.

OM : Abbreviation meaning Old Man - any male operator, regardless of age or marital status.

OSCAR : Orbiting Satellite Carrying Amateur Radio

Oscillator : Also known as a frequency synthesiser - A device that produces a signal at a particular frequency.

OCF : Off Centre Fed Dipole. A dipole that has sides of unequal length. These are multiband wire antennas