O

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

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