Ohm - The unit of measure for resistance.
Ohm's Law - Voltage equals resistance multiplied by current.
Oil - When relating to a liquid-filled transformer, refers to mineral oil.
Oil Fuse Cutout - A zero awaiting (expulsion) interrupter that is used under oil; expulsion Bay-O-Net; Cartridge Fuse.
Oscillogram - A record made of an oscillograph.
Oscillograph - A device used for recording and measuring the instantaneous values of a rapidly varying electrical quantity as shown by its corresponding waveform.
Overvoltage - A voltage above the normal rated voltage or the maximum operating voltage of a device or circuit. A direct test overvoltage is a voltage above the peak of the line alternating voltage.