OHM - The unit of resistance usually known as R.

Ohm's Law - Written I = E/R where I is current flowing , V is the voltage and R is the resistance.

Oil Trap - A device to trap oil from leaving the compressor and entering the system.

Open Compressor - A compressor where the motor is not sealed inside the compressor itself. 

Open Loop - A system where water is pumped from a water well, pond, lake or other surface source for use in a heat pump.

Output - The part of the circuit where the process signal is available.

Oversized Evaporator - A evaporator with a greater capacity than the nominal capacity of the  system. 

Ozone - The O₃ from oxygen .It's used as a odor eliminator.