OHM'S Law: States that the current in a circuit is directly proportional to the voltage and indirectly proportional to the resistance. Expressed as a formula; I = E / R

Open Center Circuit: Open in which pump delivery flows freely through the system and back to the reservoir in neutral.

Open Center Valve: One in which all ports are interconnected and open to each other in the center or neutral position.

Operational Amplifier (OP AMP): An integrated circuit amplifier with special characteristics (high gain, high input impedance, low output impedance, differential amplification) that make it especially suitable for electrohydraulic control system.

OR Logic Gate: A digital circuit in which the output state equals a binary 1 if any of its input are binary 1.

Open Collector: A term used to describe a signal output that is performed with a transistor. An open collector output acts like a switch closure with one end of the switch at ground potential and the other end of the switch accessible.

Open Loop: Refers to a motion control system where no external sensors are used to provide position or velocity correction signals.

Opto-isolated: A method of sending a signal from one piece of equipment to another without the usual requirement of common ground potentials. The signal is transmitted optically with light source (usually a Light Emitting Diode) and a light sensor (usually a photosensitive transistor). These optical components provide electrical isolation.