The term OEM stands for Original Equipment Manufacturer. What does that really mean? It means that the part was made by a company that is a subcontractor to a vehicle manufacturer. It DOES NOT mean the part was made by the manufacturer. Some examples;
Most fuel parts on a VW are made by Bosch. This means that Bosch is the OEM for VW regardless of where you buy the Bosch part it is still OEM.
Hella is an OEM light supplier to many of the European vehicles.
Almost all remanufactured parts are OEM as the original part is rebuilt which does not change whom made the part.
It has long been a common missconception that only a vehicle dealership carries OEM parts. Though they would like you to beleive that, it is not the case. Many if not most parts you get from a dealership you can also get elsewhere for less money made by the exact same company in a different box.