operational CG (weight and balance). The center of gravity of an aircraft when it is loaded for flight.
operational control. 14 CFR Part 1: “With respect to a flight, means the exercise of authority over initiating, conducting or terminating a flight.”
opposite side (mathematics). The side of a triangle opposite the angle being referenced. In the illustrated triangle, the solid line shows the side opposite the angle ..

optical coupler. An optoelectronic device used to couple electronic circuits by using electromagnetic radiation, either visible or invisible. The input circuit contains some form of light source, usually a light-emitting diode (LED). The intensity of the light is determined by the amount of current flowing in the input circuit.
The output circuit contains a photodiode, phototransistor, or light-dependent resistor. The light source and light sensor are mounted in a light-tight housing, so the light emitted by the source striking the sensor controls the output of the coupled circuit.
optical micrometer (precision measuring instrument). A precision instrument, used to measure the depth of scratches and other surface damage to a piece of material. The lens of the instrument is focused on the undamaged surface which is used as a reference, and then it is focused at the bottom of the damage. The amount the lens must be moved to change the focus is converted into a measure of the depth of the damage.
optical pyrometer (temperature measuring instrument). An instrument used to measure the temperature of molten metal inside a furnace. A piece of resistance wire extends across the optical path of the instrument, and current through the wire is increased until its heat causes the wire to glow.
The user of the instrument looks at the molten metal in the furnace through the instrument, and while watching it, increases the current through the resistance wire until it glows with exactly the same color as the molten metal. The indicator on the instrument converts the amount of current flowing through the resistance wire into terms of the temperature of the molten metal.
optional equipment (installed aircraft equipment). Equipment approved for installation in an aircraft, but not required to be installed. Several types of communications and navigation radio equipment and different types of instruments are included in the optional equipment list.