Open as Applied to Electrical Equipment - Means that moving parts, windings, or live parts are exposed to accidental contact.

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.

Optical Fiber Cable - a cable consisting of one or more optical fibers that transmits modulated light for the
purpose of control, signalling, or communications.

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.

Outlet - A point in the wiring installation at which current is taken to supply utilization equipment.

Outline Lighting - An arrangement of incandescent lamps or electric discharge tubing to outline or call attention to certain features such as the shape of a building or the decoration of a window.

Overcurrent Device - Any device capable of automatically opening an electric circuit, under both predetermined overload and short-circuit conditions, either by fusing of metal or by electro-mechanical means.

Overload device - A device affording protection from excess current, but not necessarily short-circuit protection, and capable of automatically opening an electric circuit either by fusing of metal or by electro-mechanical means.

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.