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O

Ohm : (<math> \Omega</math>) The unit for resistance

OLIVIA : A digital mode using multiple FSK carriers; intended for operation under QRP or weak-signal conditions.^[1]

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

OSCAR : Orbiting Satellite Carrying Amateur Radio

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