Final answer:
Oil foaming usually occurs in the outdoor evaporator coils of a refrigeration system, where the liquid refrigerant is cooled through expansion.
Step-by-step explanation:
In a refrigeration system, oil foaming usually occurs in the outdoor evaporator coils. The gas in the system condenses to a liquid, and the liquid flows back through a pressure-reducing valve to the outdoor evaporator coils, where it is cooled through expansion.