The work of an oil separator in HVAC
The work of a dehydrator in HVAC

I trust this is not homework!
The prime function of an oil separator is to separate the oil from the refrigerant and return it to the compressor crankcase before it can enter other components of the system. Remember, oil is not a refrigerant, it belongs in the crankcase to lubricate. A dehydrator removes water from the system by passing the refrigerant through a dryer, a water collecting chemical. If properly installed and flushed with Nitrogen, there should be little ,if any, water in the system.